Some of the issues I had with 9550 XPS:
- battery swelling
- fan spinning on high speed for no apparent reason (not due to CPU load)
- random wakeups while sleeping (happened in my bag once and the laptop almost overheated :( ). This was eventually fixed via BIOS updates
- lots and lots of BIOS updates to fix all kinds of issues- at least 1-2 every month since its initial release. Although I am glad they are being fixed
- OEM wifi card (broadcom) was a POS in both Linux and Windows (Windows came with the box). I replaced with Intel card and life has been great.
The above may not be a problem in current XPS (9660 I think), but I really don't know. The XPS forums I have been to seem to filled with people with issues and instead recommending Precisions if you are OK with a less gaming based GPU. Plus you get business support and not the craptastic consumer support. If I were to do it again and wanted a 15" I'd definitely go with 5520 precision.
RE: XPS size and weight vs Precision -> If you stick with 5000 series it is almost the same box. If you bump to 7000 series you are getting into supersize land (but you can also fill it with more goodies that of course weigh more). The only downside is that all precision laptops are 15" - if you want smaller form factors you would need to go XPS (e.g XPS 13).
RE: Ubuntu v RH variants - it doesn't matter. Go with what is comfortable and familiar to you. AFAIK Ubuntu may have a more desktop/laptop friendly setup unless you go with Fedora (which is pretty much equivalent to Ubuntu non-LTS versions).
One final thing: the one thing that the Dells have over the Macbooks is user serviceable parts. IN the 15" models, you can replace SSD, WiFi, battery, memory very easily AND without compromising weight or size. I really wish Apple would get off the thin -> super-thin -> paper-thin? ride and offer this kind of modularity again. Hell at minimum allow replacement of SSD, memory.... but that is another rant for another time :)