Opinions are plentiful, everybody's got one. Here's my 2 cents worth:
First, do your own research. Sure, they're both "evil, assualt rifles" used the world over, but I've found that they are different animals, really. There are literally millions of AK's out there. AK's are tough, relible, and decent shooters (as are AR's if you keep them clean), but in general they lack the quality and accuracy of a decent AR.
Speaking as an owner of both types [(2) AR's (1-Preban Colt, slab-sided, 20" upper, 2-Preban Olympic Arms lower, Bushmaster internals, with M4 Bushmaster upper, retract stock, etc.) and one AK (Romanian AK-74, poly furniture, etc.)], I'll take either of my AR's over the AK in terms of overall quality and accuracy.
BUT, I gotta tell you the AK's are tough little bitches. The 5.45x39 calibered AK-74 is a sweet little shooter. It does nicely out to ~200 yds, though I've never tried it beyond that. All AK sights stink (same type of sights the Reds have had on their rifles since the late 1800's - M91, M38, M44, SVT, SKS, etc. did I miss any?). Nearly all of them need trigger jobs (lots of creep, takeup) and I severely dislike their safety's design.
Depending on who you buy from, you may or may not get a quality piece, so be careful. I'm in the process of replacing the trigger parts on my AK-74 due to trigger slap (cheap euro trigger group parts, replacing with adjustable US made trigger group). Otherwise, I've been well pleased with the '74. (Got it from Tennessee Arms about 3 years ago.)
I agree with what other guys have said, buy both - BUT buy quality. Like I said earlier, you can't compare a stamped receiver AK to a decent AR, that's like comparing apples to oranges. Stamped receiver AK's go from ~$275 to ~$375 or so. Post ban Bushy's (depending on the type of upper you get) are in the ~$750+ price range. There's are reason for that and it shows immediately when you pick up the two weapons.
For an AK, I'd suggest getting a milled receiver Bulgarian (be prepared to part with ~$800 or so). These are the cream of the crop in terms of quality AK's. They price out almost the same as a decent AR.
For AR's, get either a Bushmaster or a Colt and you won't be dissappointed. Not to say that the other makes aren't just as good, these are the ones that I've dealt with. AR's have good sights, easy to operate, and are capable of considerable accuracy. They are easy to put together from a kit and difficult to screw up (although, I do not like the way some of the lower parts springs are held inplace by the buttstock and pistol grip). You might consider buying a decent lower receiver, good quality parts kit and assembling it yourself. I've done it and it's not difficult at all.
Best advice - pay attention to what the knowledgable guys on boards like this have to say. There is a wealth of info available in the different forums that can save you a ton of headaches. Ask questions, these guys will help you out if they can.
Finally, go to the range with some buddies and try to shoot as many different types of AKs and ARs that you can BEFORE you buy something.
Good Luck and I hope this helps,
Jim