As a counter, I've owned both as well and went with the Vibrant..
I'm not really sure what "better call quality" means, unless it was an issue with AT&T. For me the Vibrant has been better for calls simply because I don't have to say "What? I missed that..." Anytime I was talking on the phone in a less than quite enviroment. I'd say the X's volume issue is just as significant a problem as the Galaxies GPS issue. Both are supposed to be fixed around the same time and seem to affect users in varying degrees.
I had an original Droid before the X or the Vibrant, I was used to capacitive buttons. Having used both now, I can honestly say I don't prefer one over the other so long as whatever it is actually works, which they seem to do on both.
I didn't miss the size as much as I thought I would, as a bonus the Vibrant is more one hand friendly (important to me), and I felt the improvement in the screens contrast, color saturation and bright sun performance made me miss it even less. I do wish both had an increase in resolution tho.
The galaxies by the way have both composite out which mirrors the display on the phone as well as HDMI out via a special USB cable. I do wish there was a dedicated camera button on the Galaxies, as well as a flash as it makes a great flashlight but in practice sucks at what it was actually intended for.
Finally, My wife and I both have Vibrant's and neither of us have had an issue when using the GPS. We have both used it in the car as well as with MyTracks while biking. As for whether it will come or will fix the problem, I've seen youtube videos of a fix for the Captivate that is in testing that certainly seems to fix the issue. Not sure why they would put out a press release saying a fix was coming and clearly be working on a fix if they have no intention of ever releasing it.
Better by the way is very subjective, I think Samsung and Motorola both are very good companies in general that have had faults in their past. I'm willing to wait and see which is what I would have to do with either phone.
Overall, I've been happier with the Vibrant but enjoyed the X as well. Glad you found the right fit for you as well.