HOB filters are generally cheaper and simpler, so you can get better filtering on a budget. More bang for the buck.
But personally I don't think there is a problem with your current Eheim, it should support any sensible number of fish that you could keep in that tank.
But I agree with the others, that as adults, you will have too many fish, and extra filtering isn't going to be enough to make up for that.
Right now you don't have a problem as the fish are only babies. Next year when they start to mature.. not so good. Aggression, problems with water quality etc. So there is no need to panic, but you do need to plan ahead.
Remember we are planning on you keeping BIG healthy fish that might live for 10 years or more, not just keeping them alive till next year.
I'm planning to move my Oscar and Pleco to an 85gal soon, and I can only get away with that because the pleco is a rescue and a little stunted, like 12" after 10 years, so is not growing much more. But thats just 2 fish, and the tank will be fully stocked.
Ian