Before an appropriate answer can really be given you will need to let everyone know what type of fish you have (or plan on having) and their current size.
That being said, I use two Fluval 304 canisters on my 125, both of which have large sponge prefilters on the intakes. I currently have 45 juvenile Tanganyikans (Trets, Leleupi, Calvus, Occies, Brevis, and Cyps) in my tank. As far a biological filtration is concerned, I feel my filtration is adequate for the amount of fish in the tank...no ammonia or nitrites and I haven't lost a fish since receiving them two months ago. However, if I start to notice a rise in ammonia or nitrite levels I may have to add my spare Fluval 304 to the tank. Time will tell ;-)
If it were me, I would probably add another FX3 to your tank. I like the redundancy that dual filters offer and prefer canisters for their ease of maintenance. Using the same brand of filter is also a plus since you only need to stock spare parts for one filter, i.e. impeller, floss, sponges, gaskets, etc.
I've always hated doing maintenance on HOB filters but if I were to buy another one, it would be one of the Aquaclear filters. I currently use those on some smaller tanks I have and they are pretty quiet in comparison to other HOB filters I have owned in the past.
the only fish i have of yet are my two 6.5" severums. i am going to be keeping some SA/CA cichlids. the 2 sevs, a salvini, true parrot, a grammode, a white saum, and maybe a festae. i was thinking about a second xp3