I would stand by the Tunze due to the size constraints. That and they have been proven on this exact same tank model.
Sure there are other skimmers out there, but like @
aussieant32 has said, how much money has to be wasted just to end up with the Tunze, or more waterchanges.
I mean every skimmer has its pros and cons. Most of the time, it is the cheaper skimmers that will cause you more grief.
From personal experience and what I've read, you really can't beat a stone skimmer on a small tank. Providing you don't mind having to replace the stone regularly, getting an airpump with a rheostat, and modifying the rear wall of the tank just to make it fit.
The cheaper pump venturI style skimmers are generally hard to tune, put out way to many microbubbles, and aren't efficient at waste export to justify buying them. What keeps them around are people trying to get into this hobby on the cheap, and not researching the skimmer before purchase.
Now don't get me wrong. There are some stunning tanks done cheaply, but finding these are few and far between.
Now the question would be, are you willing to save a few dollars and try something a lot of other people have tried just to find it doesn't work as it should Or go the tried and true way that a lot of people wished they had done at the start.
Like I originally said, for what space constraints you have, the Tunze it's the only way I would personally go. If funds didn't permit, I would rely on waterchanges.
Another thing you have to remember though is a skimmer doesn't give you a green light to lax on your waterchanges. Whether you have a top dollar skimmer or a cheap one, nothing beats nutrient export like waterchanges. Skimmers are for getting crap out of the water, not replacing the elements that are used up.
Another mistake is to think "That's ok, I'll dose all the elements that are low". Yea that will get all the elements back up, but that's only for the elements we test. there are still a lot of trace elements we don't test for that will need to be replenished.
Sorry for the extremely long post ( I think its my longest post on this site :p ) but I had to put all this across so you have an informed decision rather then letting you do the same mistakes I did when I 1st started (yes I was one of the poor buggers that got a few of the cheap skimmers).
I hope you'll find some of this info helpful.