if you plan on keeping an oscar and a pleco I would go as big as possible because both fishes create a lot of waste.
Eheims and Fluvals are both pretty good. I personally prefer the Fluvals over Eheims because they are a lot cheaper where I am. However, they are pretty close to each other in quality. Eheim usually do a ceramic impellor shaft, whereas Fluval normally do a stainless steel shaft. The ceramic can break more easily but doesn't wear as readily. I found Fluvals a little easier to get appart and clean but it depends on the actual model.
Your best bet would be to go into a petshop and get them to show you both models and see how easy they are to take apart and clean. You can also look for spare filter sponges to go in them to help trap more gunk from the fish.
