Well first, I'd like to say "welcome to the dark side".
When it comes to chosing the right tank there are some key things you'll want to remember and consider beforehand...
1. Footprint is vital when you're talking salt, as nearly everything you place into that tank will be living - from the sand, the rock, the plants/corals, fish, inverts... therefore, you need to be able to accomodate basic necessities of any organism you intend to keep.
2. Chemistry in saltwater is a whole other beast than fresh, and in smaller tanks, yes it's much harder to regulate. This isn't because the "chemistry" changes, but rather because you are dealing with a much smaller volume, and therefore, much less "wiggle" room when toxins are introduced (IE: Ammonia, nitrates, nitrites, etc.). Smaller tanks require much more maintenance to keep algae's at bay, regulate temperature fluctuations, maintain optimal salinity levels, as well as dosing of supplements/foods/medications, in addition to removing detritus and other solid food/waste particles within the water colum as these - if left unconsumed, will poison your waters in no time.
3. In smaller areas, it goes without saying that only smaller animals will thrive. Having saltwater tanks under 40-55 gallons alone already significantly limits your choices on livestock, but opting to go below 10gallons pretty much seals the deal with a meager handful of inhabitabts to choose from.
4. You always want to anticipate growth. It's OK to start out with a bigger tank than you can fill, and it's OK to have it look "empty" for a while. It's all about growth as a keeper, and expansion in habitats, so starting big is never a bad thing. Starting small however, can very soon frustrate you not only because it requires a great deal more of time and effort to maintain (and I have seen too many give up on salt because they start too small), but also because once you really get the hang of things, and become excited, you realize that you are so limited because of the size tank you have. It's easy to up-grade, but it's going to mean purchase, and set up, and cycling, and stocking of an entirely new system, of which then, your smaller one will become obsolete.
My suggestion to you, is to stay away from the Nano/Bio cubes, and either use the 25 you've got, or start looking around online (at any retail supplier or craigslist, ebay, etc), for larger systems. You'd be supprised when it comes to space as well - (space it takes up in your home, I mean). Get a tank with a cabinet stand to go under it, and not only can you store all of your equipment (scrapers, tubing, syringes, clips, scissors, etc, etc, etc), but also all your supplements, and bigger pieces like the Sump/Canister/etc. This keeps it clean, organized and space-saving for smaller areas, like it seems you might have.
Ultimately it's up to you, but IMO and from my experience and what I've watched others go through (Thus the reason I advise against it now), is to go as big as you can right now, within budget, and slowly get your feet wet in this other world we call Saltwater, and you'll be happy you did 6 months from now when you see how many more options you have, and how much easier it is, to find your maintenance "groove" and keep everything top notch.
