Whatever you do, do not get the PC version of SR2--it's not very compatible with a lot of devices and it's regarded in the community to be fairly unstable. Stick with the console versions and you will be fine.
That being said--for me--I really enjoyd both games quite a bit and for different reasons.
I enjoyed GTAIV's more serious take on the open-world genre as it trys to tell a more serious and compelling story filled with much (MUCH MUCH!) more interesting characters than SR2s, while it attempts to move the genre forward. I also loved the overall detail in the world--it's stunningly detailed in its design (I would like to point out that when I say detailed I'm not necissarily talking about graphical fidelity--but more so of the design, layout and architecture design and detail of the city!)
No alley looks cookiecutter or a copy & paste job from another part of the city. I know it sounds ridiculous to mention but even some of the clutter in the alleys, the steps leading up to buildings and curbs running along the backstreets look unique in their own right. It's totally fascinating to me to look at these things, but I'm also a little weird too.
But wait! I also loved SR2! It's really, sort of wacky and fun ride with a totally different spirit to it. It's not trying to move you with it's narrative (or acting chops). It's there to say "Ok, so here is our laughabily serious plot--now go outside and play!", and you run off filming a reality show (cops), being a body guard for celebrities or even chasing streakers. It's great fun, so it depends on what you want out of the game.
I didnt finish the game (got about 3/4 of the way through it) where as GTAIV I saw it through to the end (beat GTAIV twice actually). SR2 is a great game and most certainly worth whatever the price of admission is these days.
Oh and one last thing, in terms of GTAIV
Unfortunately there still remains one absurd design choice that drives me to shouting at my TV in I hopes that Sam Houser will take notice--scripted phone calls. They have got to stop.
Actually the calls wouldnt be so bad if Niko or Johnny didnt stop running to answer these intrusions. But in regards to this game in particular, I dont know of anything else more frustrating than the inevitable phone call once you've completed a mission. The likelihood that one is involved in a car chase in GTA involving a insane amount of munitions had got to be high. Not only are there bound to be 37 cop cars hot on your trail, but you are most likely going to be running from your burning charriot thereafter. To have the protagonist suddenly stop running while trying to get away is maddening. It got to the point where I would try to find an alley to run and hide in so Mr Turtle wouldnt get killed. It wouldnt be so bad if the save system was a little more logical, but after a long tedious mission this is the last thing we want to have happen.
More here for those that are interested.