I agree, I like Eheim, Fluval is ok, and I have to say, after almost 10 years my magnum 350 is still kicking like a 20 year old.

Maybe this will help:

When I ran my 55 planted tank with an arrowana (that was getting to big for his home) I ran two magnum 350's. Loaded the media cannister with 50/50 carbon and amonia absorbing mix, and had their returns through the bio-wheels. For a while I also ran Emperor 400's, but found that having a grand total of 4 bio-wheels operating on my system out-gassed to much of my precious C02. If you are not going to be injecting CO2 and growing plants, then you don't need to worry about that.

Oscars are messy fish, so I would say err on the side of caution, buy more filter than you think you need. And make sure it is EASILY accessible. You will be cleaning the sucker so often you don't want to provide any excuse not to touch it.

Also, what type of 55 gallon do you have? Mine was a show tank, and very rapidly my arrowana outgrew the front to back length, it became difficult for him to turn around, oscars are not nearly as nimble as an arrowana so if you have not bought yet, I would encourage you to find a 'thicker' design, especially if your going to aquascape with lots of plants, wood, and rocks. I made the mistake of assuming I would be financially capable of upgrading my arrowana's home by the time he got there, my fiancee (at the time) and her engagement ring/wedding band / wedding, honeymoon had different plans. Make this upgrade or new purchase for your oscar the last one you have to make, and get an appropriate sized tank for him/her. (Not saying your 55 won't work... Just that I don't think mine would have.)