I enjoy both 3.5 and 4th for a multitude of reasons. Both 3.5 and 4th can offer what your looking for. 3.5 having much much much more flexibility and much more grittyness. But with a few simple rule changes can make 4th gritty as well very easily, I sadly in 20 years of RPGing have never played Rolemaster one of the few I dont think I have..
3.5 was phenomenal, our group could play for hours and argue over rules for hours, the thing about 3.5, is whats in 1 book, is contradicted in another book, and each new book just trumped what was previous in other book. It was like they just wrote a bunch of **** shoved it into the printer and then errata'd it all 6 months later, then put out another book republishing the same feats/classes over again with new update's its like really?? Q/A testing at all? Do you have any or do any? Its really bad if you have 'power gamer' in your group, and make a cleric with nothing but Extend Spell Persistent spell Divine Metamagic: Persistent Spell and Extra turning and crap tons of those stupid rods that gave you 4 extra turning attempt's. Then have silly stupid cleric running around with 24 hour Divine Power/Righteous Might etc... Out meleeing the fighter with a highter attack bonus a higher strength bonus etc.
Just about anything was possible in 3rd, but its so easy to break...so very very easy. We have been running the same 3.5 campaign since it was relased, I think we are on death # like 415 but its fun, usually 1 a year we have our Epic tournament as well, DM sets a level convert your favorite character to that level and do battle in the Arena!
4th just works, there is less going through book's we have a simple rule, besides Eberron and Forgotten Realms if its in the builder print it out and you can use it. I absolutely LOVE the 4th edition Character builder, the web based once its fixed and not made to the iq of 4 I might enjoy it
You can find offline versions of the builder on your favorite torrent site really great tool for just getting an idea of wht 4th is, if willing to spend $10 can sub to the D&D insider for a month which will give you access to just about everything except for the actual books.
4.0 though is not gritty at all, unless the DM is just brutal, its pretty much impossible to die in 4th. The only times we have died so far has been death trap's and when the Red Dragon drug my rogue into lava
Big complaint I do have about 4th as well, is Monsters have so many fricking Hit points and do so little damage, but we also dont have a High damage striker in our group.
If you are a heavy roleplaying group, either one works, 4th is pretty hard to do with out tiles/map's mini's as everything is expressed in squares speed 5/6 6 square's, if you want tactical combat, i think 4th does a much better job of that than 3rd did, combat just flows much easier, much quicker in 4th than in 3rd.
4th is kinda best describe I think as a Table Top MMORPG, classes have roles know, Fighter/paladin/Warden/Battlemind are defender's aka Tanks, Rogue/Ranger's/Sorcerers/Warlocks/Avengers/Barbarian's/Monks are Strikers aka DPS, Wizards/Psions/Druids/Invokers are Controller's, Cleric's/Warlords/Bards/Shamans/Ardent/RunePriest are leader's aka Healer's/Buffer's. Its what they do, Wizards dont have nearly the verasatility they use to, which I think is good and bad, good because you can no longer teleport wherever you want whenever you want, same with planeshift etc.
Basically spells like Teleport, Knock, Comprehend Languages, are all rituals now, they all cost money to cast and have a minimum casting time of 10 minutes at the absolute minimum, so no more teleporting out of middle of combat, or planeshifting wherever you want.
But at the same time, Im just now getting the ability to Fly with my wizard at 17th level!! Its alot harder now, when you see a pit trap to just go ok memorizing fly, flying the party pass pit trap. which I also very much enjoy, I love coming up with crazy idea's to beat our DM's trap's and he loves his trap's! There is skill challenges now, so say for instance, there is a pit with like a 3" rail going across said pit, player A goes across pit, SKill challenge ensue's, first make a balance check (Athletics? Acrobatics? whichever skill it is) fail lose a healing surge (1/4 of your HP's thats how you heal know, spend surges and certain classes give you bonus so you heal 1/4 of your HP's + whatever bonus from said class), dont lose hp's just 1 from your surge value (Leaders get more, strikers get less) you run out of surges that's when start losing Hit points.
So it is kinda simplistic game, but I have fun every time we play. I can absolutely recommend 4th, I think I do prefer it over 3.5, just how much smoother it flow's and the fact I dont have to look in a book every 5 flipping minutes, everything I need is on my sheet, besides my 1 printout page of status effect's.
Forgotten realms was utterly annihalted with the 4th revamp, I refuse to open the book or look through it anymore, we are using our own custom world. If your a forogtten realms fan, stay out of the 4th edition faerun books.
I hope that makes sense, Im tired