If your 37g is 30" long, then I'm not sure if you can fit it along with your Top Fin 40. Consider a Aquaclear 70 or 50. They are smaller and you will have more room in the back of your tank.
I have a Penguin 350. Its about 16" long. I also have a AC
70 hanging on another tank. Here are the difference:
1. Penguin a lot bigger
2. Penguin only 50gph stronger (very slight difference IMO
3. Penguin filter pad is included, but you will have to change it since charcoal is inside the filter pad. You probably have to make your own filter pad as well. The AC
has layers of filter media, and you do not have to buy anything additional.
70 is about $5-$10 more expensive.
can adjust the flow. You might end up with too much current with the Top Fin 40 + Penguin, but no way around it. With Top Fin 40 + AC
, you can decrease flow by sliding a switch on the AC
Both filters are pretty quiet. I don't know the wattage of each filter.
Its your decision. I'm selling my Penguin 350 and keeping my AC