The sunsuns are rated mainly for the uk 220v, but work on 120v and have a 2 prong cord. The aquatops have a 3 prong cord and are rated specifically at 120v. I have also heard that the plastic canister itself is flimsy on the sunsun, but I haven't been able to verify that since I have never seen a sunsun in person. Other then that I believe they are exactly the same.
Actually I have a sunsun 303b w/uv running on my 55g and it came with a 3 prong plug and rated for US 120volt at 60Hz (I'm an Electrician by trade so I do know the difference). As for the plastic of the canister itself it's thicker then the plastic that was on my old Fluval 305 canister and the latches are much more robust then the old Fluval / Eheim style latches from about 15 years ago. The handels on the media trays do leave a bit to be desired the handles do tend to pop off pretty easily but the media trays can be removes with out them just takes a bit of side to side shifting then it comes right out. I've looked them over side by side since I know someone with an Aquatop canister and they are virtually identical some cosmetic color differences and sticker colors as I mentioned previously but they are the same as I can tell with out dissembling them any further.
I researched canister filters for a good month before deciding on the Sunsun I personally saw no reason to spend almost double on an Eheim / Fluval simply for a major brand name emblazoned on the side of my canister that will be hidden from view under my stand. Not to knock the quality of higher end canisters I know they work very well or they would not be so popular but as with all things in this hobby the manufacturers can and do charge extra because they can this is a nitch hobby although very popular it still occupies a relatively small segment of the overall population so if it goes in your tank they charge extra, for example a regular 20oz paint ball tank sells at wallmart for about 20$ that same tank labeled for fishtank use at the LFS retails for 80$. this happens over and over again don't get me started on the cost of T5HO tubes at the LFS swing by an electrical supply house if you want to see the price difference. Or the ridiculous markup for LED fixtures, 400$ for LED's I built my own for under 100$ puts out about the same amount of light and uses less power overall and I can add to my system down the road if I so choose.
But I digress Canister filters are all very similar as long as they get the job done and they keep the fish happy and don't leak that should be all that matters I just see no reason to spend almost 200$ on one when the 70$ model does the same job leaving me cash in hand to get more Flora/Fauna.