I'm in the opposite situation. I started gaming two years ago with two different groups. One uses 3.5 D&D and the other RM. I've been trying to convince the RM group to switch over to d20 for a while.
RM isn't exactly more realistic than d20. Admittedly there are rulles for pretty much EVERYTHING.This can be a bit problematic though, as games can come screeching to a halt while someone looks up rule X about how the Hygiene skill applies in situation Y. If you can think of a skill it probably exists in RM. For instance, in 3.5 D&D, Sleight of Hand woud let you pick-pocket, juggle and do card tricks. in RM, you would take each of those skills seperately. Is it mroe realistic to say that a master pick-pocket might not know how to juggle? Yes. Does it matter? I don't think so. Frankly who gives a ****? Certain skills are just so useless that no one would waste the ink to write them on a character sheet. Who is really going to waste skill points on Hygiene?
Combat also takes this approach. In RM, a 1st level goblin armed with a salad fork could potentially take out Fragdor the Smasher, 20th level Fighter. The reason for this is critical hits. It's been my experience that HP damage is rarely the deciding factor in RM fights. More often, you roll high enough to get a critical and then if you (or a foe) are lucky enough, that crit is an instant kill. The criticals are rolled on d100 chart with a severity rating from A through E. It is realistic, that goblin could get lucky and jam his fork right into Fragdors jugular and kill him instantly. Is this a good thing? Depends on what side of things you're on. It is nice to take out a high level foe with a lucky shot, on the other hand, it sucks to be killed in the same circumstances. I could go on at length on the combat system but I'll try and sum it up.
1: Realistically being killed by a lucky foe sucks. The combat system adds a high degree of randomness and that works in favor of weaker characters/enemies. I should add too that having a significant chance of dying in every encounter is a bad idea when the character creation system is so time consuming.
2: Stuns. Some critical hits lead to stuns (Can move 25%. No attacks. Possibly no parrying) and frankly, this is an enormously bad idea. Stunning can throw the fight heavily in either direction and it sucks to have to sit out most of a fight, again, because someone got lucky on their critical roll. More randomness.
3: Experience. The bulk of XP is earned by actually being the one to bring down a foe. Who cares if someone else spent the entire fight working it down? If you get the last hit, you get the XP. There are other ways to earn XP but they don't make up for this. My group had a long discussion on this and we moved away from the kill-based XP system when we realized that nobody was willing to heal or play a support character as they would be losing out on XP.
4: Injuries. yes it is unrealistic in D&D when you someone with 1 HP out of 100 fights as well as someone at full. Yes it is unrealistic that with all that damage, nothing vital has been hit and you take no penalties. On the other hand, having specific injuries (These are also obtained on the critical table) such as tendon damage, severed leg muscles or broken bones means you have lots more to keep track of. Furthermore, healing for such injuries is almost never available. Better learn to live with that missing nose.
This is turning into a bit of a rant so maybe I should just sum it up. If you want realism go with RM, if you want a well balanced and easy to play system stick with D&D.