I had the 14gallon, and i thought the small size was nice. But, since it is smaller the back chambers were more difficult to work with.
The 29 is nicer since it has a canopy lid for the lights/front of tank, and a separate opening for the back chambers. It is larger, so you'd be able to keep more things in it.
What do you have in your 55g reef? Depending on your current livestock, you'll have to make adjustments to the biocube since it's stock lighting won't be powerful enouh for pretty much anything but softies.
A lot of people run their biocubes open top, and just get a fixture or hanging pendant. I did it with my biocube 14 until i upgraded to a solana. Running it open top makes maintanance easier, but you'd have to spend more money on a good fixture.
I'm not sure where you'd get it, LFS or online, but MarineDepot's pricings are:
1) biocube 14 - $179
2) biocube 29 - $255
3) biocube 29 HQI - $400
None of those prices include a stand; you could make, buy, or not need one. I think the biggest biocue difficulties are lighting and maintenance, and the 29g makes working in the back chambers easier since their larger. However, you could get the 14g or the 29g and run it open top with a separately purchased light fixture. Their HQI model of the 29 has a 150w 14k halide, so you'd have strong lighting to keep a variety of livestock.
I think that your best bet would be, if budget permits, buying the HQI biocube, or getting the 29 or 14 and running it open top with a more powerful light fixture.
my 2 cents.
-Austin