I have an XP2 & an XP3 filter running on my 100 gal. cichlid tank and love 'em. They are very low maintenance and provide good flow. Check into them. I think you will like them.
I would get two canister filters that house media such as ehiem or fluval. A cheaper alternative would be to get two hang-on filters such as an Emperor 400 or Whisper 60. This way you won't upset your water chemistry too much by alternating filter cleaning.
p.s. i never understood the purpose of undergravel filters they just create problems later...
I agree with managuense. Use a canister and a HOB. everyone will tell you eheim and i agree if you have the money. I like the filstar xp3 and the vivaqua filters because i can afford them.
I prefer Filstar over Eheim simply because of the price, other than that they are essentially the same thing.
A Filstar xp4 and an Aquaclear 500 would be excellent for your 120. Unfortunatley my heavially stocked 110 is underfiltered. I only have an internal fluval 4 and an eheim 2226. which I got used on Aquabid.