The open comb version is the R41Go with the Muhle R-89 (or it's close cousin the Edwin Jagger, they have the same head). It's one of the best razors ever made hands down. Finish, and balance are superb, and it shaves like a dream. It always gives me smooth comfortable shaves.
I don't think it is hugely aggressive, and based upon what other posters have said, it should be the same level of aggressiveness as the Merkur.
I think you should clarify which Muhle R89 you are referring to, the old one with the open comb (which was said to be madly aggressive), or the newer one with the standard safety bar (which I think is fairly mild). I don't think I have heard anyone who owns a Muhle R89/ Edwin Jagger DE89 say it was aggressive. Fairly mild seems to be the consensus.
I agree with the sentiments expressed here. I have a 34C and an R106 (just the R89 with black resin handle). You can't go wrong with either.Welcome to B&B!
As for the 34C/R89 razor dilemma;
In terms of aggressiveness there is no crucial difference IMO; the 34C is a tad more aggressive than the R89 but not so that it should be the reason to choose one over the other.
In terms of quality the R89 is clearly the superior product with a much beter fit and finish, but the Merkur is no trash and will meet the standards for everyday use easily. But where the Merkur is well made, the Mühle echoes the craftmanship of yesteryear and is indeed making the Merkur look like a factory's 2nd choice.
Most important difference is in the size and weight of the handles. About the same in diameter the Mühle handle is longer and lighter and thus the balance of the razors is very different. The Mühle is a little head heavy and lighter, where the Merkur is heavier and the center of gravity is more towards the middle of the razor.
This is something that has nothing to do with good or better; it is just a matter of personal preference.
For my (big) hands the handle on the 34C is usable but I clearly prefer the one on the Mühle. So my vote would without a doubt be the R89 but keep in mind that the 34C has made a lot of guys happy too.