My buddies and I just went through this. I ordered an XQ38 first and it is a very nice scope, I was impressed with it and the image is very good but I was really curious about the XP50 so I ordered one and gave the XQ38 to my buddy. The XP is noticeably clearer and has about 25% more FOV. The XQ has .5x more base magnification but I don't really notice it personally and I shoot with the picture in picture anyway. I could be wrong, but side by side the PIP image seemed clearer in the XP even when they were both set to 4.2x. I find the extra resolution makes positive ID much easier on animals the size of a coon/possum/cat. The XP was about 50% more money but I don't regret buying it because I plan to use it a lot and I have no immediate plans to buy a hand held scanner.
Make no mistake though, the XQ38 is no slouch and I don't think anyone would regret buying one, especially if you are buying a hand held also. Personally I wouldn't buy the XQ50 because the FOV is so narrow unless you have a hand held. The XP won't do anything the XQ won't do most of the time, but the XP just does it better. It's like regular TV vs HD TV.