I have both and I prefer Eheim. It's just a better made product compared to XP3. I will list what I like and dislike about both.
XP3 paid $100 brand new and only come with foam pads. I had to pay another $15 for bio media. Current age of filter is 3 years. After two year of user I upgrade the intake and exit to with an Eheim kit. The performance of the filter improved a lot.
Good:
-cheaper than Eheim
-reliable
-easy to setup
Bad:
-Intake tube keeps disconnecting
-ugly color intake tube
-intake tube is too restrictive for water flow
-no auto prime
-plastic is a bit brittle but should be ok as long as you're careful when servicing
-quick release can be tricky to operate or else you'll break it
Eheim 2026 Pro II. I paid $180 online which came will full media kit plus an intake kit as a bonus. Current age of filter is 3 years.
Good:
-auto prime worked great
-easy to setup
-come with full media so no need to spend extra for media
-more quiet than XP3 even though XP3 was actually pretty quiet.
-high quality plastic
-nice design that actually worked great
Bad:
-Higher price than most filters but worth every pennies
-quick disconnect is also tricky to operate
-suction cups from Eheim do not last as long as the one from XP3 but they do work better than the one from XP3 when it was new.