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what do you mean by overly complicated with stats? like a s**t load of different stats to remember or what?

I assume you like all 4 equally, or you wouldn't have a problem, so I'll list some pros and cons:

Imp Guard -
Pros: You can outnumber even Tyranids, and you have some of the best ordinance you'll ever see. Str 8 pie plates take out huge chunks of Necron lines and will instakill Space Marine characters. Doctrines mean that your army is very customisable, and you're the epitome of the the "make 'em roll so many saves they have to fail some" strategy.
Cons: You can outnumber even Tyranids. The double edge to this one is that you have to buy, assemble and paint all those minis, which can be a reall chore and drain on your wallet. No matter what you run, be prepared to shell out chunks of cash. Also, your save is 4+, at best. More likely 5+. Dive for cover, or be prepared to die in droves. In fact, be prepared for massive casualties anyway.

Eldar -
Pros: You will have the most durable vehicles in the game. Now that skimmers cannot be penetrated when moving over 6" a turn, Eldar upgrades turn thier tanks into unstoppable monsters nearly as durable as Monoliths. Your troops will be very good at what they do, and you will have boatloads of AP 2 and 3 weaponry, perfect for taking down Marines & Necrons. You'll be able to hit hard and make it hurt. Howling Banshees always hit first. Not just strike at I10, always hit first. And they all have power weapons. Eldar are par excellence at taking down Marines.
Cons: You won't be able to take the counterpunch. Unless you plan to play Biel Tan or cheese out on the 5 Wraithlord army, you can't reliably take return fire/assaults. Howling Banshees might all have power weapons, but thier only str 3, you won't be able to touch a Carnifex in close combat. Your basic trooper is currently crap, as he gets mowed down by bolter fire and pulse rounds.

Tau -
Pros: Longest range basic weapon in the game, nothing under Str 4 in thier armory, and the basic weapon Str is 5. The Railgun is the best anti-tank weapon in the game, bar none, and they have the heaviest skimmer in the game short of the Monolith. If you so choose, you can move your entire amry like the wind, and change Crisis suit loadouts from mission to mission. With all the AP 5 weapons you have, you will absolutely rock Guardsmen, and Eldar aren't too happy either. Also, the Tau are heavily rumored to be getting a new codex in April/May of next year.
Cons: You cannot do anything in close combat. Photon grenades and a 4+ save mean that you can sit in combat for a long time, but unless you get really lucky, or have overwhelming numbers on your side, don't expect to do anything worthwhile. Also, with a races this dedicated to shooting, you sure do miss alot. Just about everything other than the tank or the commander only has BS3. Also, no Ordinance. At all. This may not seem like much, but Ordinance is an anti-infantry weapon par excellence, often either very high str, or high AP, but the closest tau have is thier direct fire submunition, which is only Str 6 AP 4. You have very few AP3 weapons, none of which are available to basic troops, and two of which require an expensive unit of Pathfinders to be effective. The Tau have NO reliable way of dealing with Necrons, and Marines are almost as tough. Also, the rumored codex. It will introduce new units, and possibly institute rules fixes. Which may change how the Tau play.

Take what I say with a grain of salt, as I only actually play Tau. The other notes are what I have gleaned playing against those armies and from comments from other players. Pity I can't say anything about the Witch Hunters, as I have not played anyone who uses them.

I like you. You know what you're saying, and you seem intelligent while doing it.

From your advice, which I do so thank you for, I think I've narrowed my choices down to either the Imperial Guard or the Tau.

If you do not want to worry about stats so much then just play an all infantry Imperial Guard army. Human stats are easy to remember, the only thing you would have to remember is what they are armed with. I play with one myself but more specifically because it fits a theme.

Don't worry too much, it's not like it's that difficult to remember what you armed each squad with as long as you made them WYSIWYG. I know a guy who just takes a bunch of infantry platoons, and gives each squad a rocket launcher and that's it. He also has a couple of autocannon teams and a lascannon team. Guard do have the ability to have 8 or 9 different statlines in an army, but you can also have as few as 2 statlines. You can make it as diverse or simple as you want.

Honestly, my recommendation to you is that if you want to start an army NOW, go IG. Because buying Tau now would be a little silly. It's been confirmed we're getting several new models and a new codex in 6-7 months, so if you want to play Tau, either only build a small army of 500-1000 points, or just wait.

rolleyes Unless of course, your opponent is Iron Warriors, Necrons or Tau. In which case he'll dance for joy and smear your tanks all over the board in an orgy of glancing attacks or high Str fire.

I personally use Chaos Space Marines (Emperor's Children), Imperial Guard, and Tau. As i've said before, Tau were my first army, and they do require strategy. The only real difference between Tau and IG are the numbers of models and heavy weaponry. I love the heavy weaponry of the Guard, but Str 3 weaponry is not fun. You do get a lot of shots, but against Marines, you need a 4+ to hit, 5+ to wound, and then they get a 3+ save. Very hard to take down. I just returned from a 3 month vacation from my Tau, but since i've started using them again, I love having str 5 weaponry again. There aren't that many heavy weapons, but I think you can get yourself into better situations. A 2 man Broadside team and a Hammerhead are a mean tank hunting force. My stealth team is pretty much my favorite unit. They've takin out 5 man terminator squads the turn they deep striked in, plus the "pop out from cover, shoot, pop back into cover" tactic is about the greatest, because aside from keeping your men safe, it annoys the opponent horribly. If you switch from Tau to a marine army, you'll be in bliss for a while because of bs4 and 3+ saves, but there are things to the Tau that are just better. Plus, with the new stuff coming out, like a marine hunting fast attack unit, and hopefully some more suits, its just gonna get better.

In case you don't feel like reading all that: Tau