I think the 85 is better wide open and much nicer than the 50. I loved shooting it wide open. Once stopped down, they were fairly similar. However, on a 1.6x focal length camera, I actually used the 50 more becuase it was the perfect length for me and albetly sold the 85.
Looking at the lenses you have it seems to me that your key requirements are better low light performance and perhaps a nicer bokeh. Would that be right?
If that's the case I think it might be worth thinking about the low light situations that you are most likely going to shoot in. Is your home likely to be scene of a lot of such shots? If so you might like to work out how much light you can get at various places in the home at different times of day. And also consider the amount of room you have and the sort of framing you could get in those places.
Although the high iso performance of the Canon cameras is about as good as it gets at present I still find a big difference in quality between iso400 and iso800 and again when you go up to 1600 (I have a 20D). If I can find a way to get to 400 I will.
Now both the 50 and the 85 seem to improve in IQ stopping down a bit. 85 (if it hits focus accurately) is a little sharper wide open (but its wide open is 1.8). In fact IQ is very very good on both once they reach f4 so for general portraits either is fine (but then so is my 24-70 zoom at that f stop - another story!). Both are still useable wide open for many applications.
One factor I've been working out for low light is the slight difference the focal length can make when taking candids of my kids handheld in low light. Basically I feel I can hand hold the 50 between 1/60th and 1/80th of a second and be fairly confident. The 85 seems to need at least 1/100 and preferably 1/125th. YMMV.
But at the same f stop that normally means another stop of ISO and the resultant drop in IQ. So although at the same ISO i would probably give the edge to the 85 if you have to use a different iso the balance tips the other way. If all your shots are on a tripod or you have really good light this may not be an issue. And 1/60th is as slow as I go for kids.
I was thinking of selling the 50 and a couple of other lenses to buy a 35 f1.4 but after upgrading the lighting in one of our rooms to get me close to an EV of +7 I may hold off. Here's the last frame of 2005 - just a fast proof from RSP as I havn't had time to PP yet. Followed to some links to other low light candid shots (snaps?) with this and the 85
50 f1.4 at f2, 1/60 and iso400 Lighting two flourescent 6500K strip lights (see catchlight and a little window light from his right. WB only in RSP and fast proofed to jpeg for web (default sharpening applied)
Here's one at f1.6 I think iso 1600 (may just have been 800) by window light and I can't remember if I used a reflector here or not (i did on some from this session) Couldn't have got this framing with the 85. I left the WB alone to catch the warmer light. May go back and play with it though.
You may not be able to tell everything about sharpness from these web sized images (before some one else comes in and points this out ) but I hope it helps your thought process.
I'm glad you decided to get the 85/1.8. It's easily my favorite lens for portraits. Keep the 50/1.8. It's a great little lens for taking pictures of more than one person inside or out. I use my 85/1.8 for single portraits and if I have room, couples. But the little 50/1.8 is where I go for portraits of two or more people.
I used to own the 50/1.4 and sold it because I there was hardly any difference in my images from the 50/1.8. In fact, the 1.8 was sharper at some apertures and had better color. Sooooo. . .keep the little 50 and get the beautiful 85/1.8. Learn how to use it and it will be with you for years and years. Some of my favorites from the 85mm:
I played around with the 85mm this week and realized why it's so appealing esp. on a 1.6X frame. Try this, turn the camera portrait and hold it to your right eye, now keep both eyes open and focus forward until your eyes can match both eyes' images (stereo-optic) and you see they line up perfectly. In other words, the frame that the lens sees is almost exactly what your eyes see. I tried this with my 50mm and it doesn't line up. Interesting...
) but I hope it helps your thought process.