Unfortunately many books about fish have incorrect information. I've read a handful of books about Bettas and other fish and I always see tons of wrong information. I've never heard of undergravel filters being bad for fish with long fins, I've never had a problem with them and I've never seen or heard of anyone complaining about having any problems with them.
Any type of filter is better than no filter at all. Since this thread is about a sorority tank we don't have to worry about finnage anyway. In a sorority tank you really should have some sort of filter whether undergravel or mounted, there should be some sort of something in there. A true filter, like Fisheh said, would be the best since there are multiple fish in the tank.
It's all up to the OP on what filter to get but I do love the Whisper filters. I have three of this filter (http://www.walmart.com/ip/Tetra-Whis...-1-ct/10291764
) in my tanks, it's super quiet. These tanks are actually in my bedroom and I don't hear them at all at night when I go to sleep. They have adjustable settings so you can have it on high or low or inbetween so it doesn't throw your fish around. I don't know what size aquarium the OP has but if it's larger than 10 gallons they have one that looks exactly the same but is slightly larger and is good for I believe up to 20 gallons. It's a fairly inexpensive filter and it's filter pad replacements are cheap as well.