I'm not very knowledgeable abouth Angels but I can tell you a group of 7 to 9 Bolivians will be fine in a 75,.....some things do depend on other tank inhabitants of course. If you chose to go with a larger group of cories I suggest 6 Bolivians. But,....the question was about filtration!
Sooooo there are a lot of diferent types of filtration, sponge filters, HOB's, canisters, sumps,..... Every system does have it's advantages and disadvantages. You will get a lot of diferent answers on this question and to say what filter is the best is just not possible becouse there are many way's to accomplish the same effect. Thing to consider are:
Flow,.....do your fish like a high flow or not
Bio load,...with a large bio load you need a large quantity of filter media
Space,.....is there space for the filter system (a sump or an in-tank filter)
Risk,.....do you want to take risks with leaking hoses / overflow / seals
Product qualety,.....some product do look the same but there is a huge diference in qualety.
So I use canisters and build several in-tank biological filter based on sponges. The DIY in-tank filter is easy to build, no risks on leaking, reliable, low maintenance and cheap. It cost a pump, some glass, silicone and sponges. The only drawback is that it will take away space in the tank.
Eheim canisters are reliable, quiet, and spare parts will be available for a long long time! There is only one type with some problems and thats the Ecco line. The bar is the weak spot on this one and can break if you use to much force. I have 2 of them and one of 8 years old that need to be replaced. I can order the part easely so not a big problem. The Eheim Pro series are excellent canisters! I like the Pro2. A very good alternative are the marineland canisters! They seem to be a cheaper but reliable alternative. If you plan on a CO2 system I suggest to skip the HOB. This creates a lot of movement on the surface and becouse of this a lot of CO2 will escape the water.