I have a Finnex 360 running in a 4 gallon tank.. I wouldn't put a betta in there, the flow is too strong. I keep shrimp in that tank, no fish. I love the filter.. it really does a great job of keeping the water crystal clear, and my plants in the tank are doing really well with the flow it provides (I also inject CO2 in that tank).
I have several Zoo Med 501's.. I have two 10g tanks with them and my petco 6.6 has one with a modified spray bar (I drilled the holes a little bigger - this slows the flow a lot). If you get the turtle version it's only about $35 on amazon.. I think the only difference is the aquarium one comes with the hanging bracket that sells for about $12, otherwise they are the same and you can save yourself $12 if you don't need the hanger :) The flow is a little gentler from this one than the finnex since it puts out 16gph less... plus 9mm lily pipes are cheaper than the 13mm, and something else to consider is the HEIGHT of those pipes.. the only 9mm pipes I've found are short, but I can't find any 13mm besides the Cal Aqua (which aren't in stock anywhere) that are short enough to fit on my 4 gallon tank.
As of right now I don't have lily pipes on any of my tanks.. it doesn't bother me that much to be able to see the filter hardware. I am going to be putting them on my 4 gallon as soon as I can buy the cal aqua pipes though.
I would really not consider the sun sun for a tank taht small - yes you can restrict the flow but you're really not supposed to go down more than 50% and that is still a LOT of flow in a 5 gallon tank... the 501 or 360 would still be a lot of flow for a betta tank and to be honest I'm not sure it would work... it's fine with my plakats in their 10g tanks, but they don't have extra finnage to deal with.