The floorspace in the 40 breeder and 55 are actually very close - the 55 is only SLIGHTLY smaller in that regard, but is pretty much just longer and a bit skinnier.
So it really depends on location, what you have room for, and what you want to do with aquascaping. You can increase usable room even more by having hides and plants that are good for walking on or climbing in. Mine has a rock cave and is as often on top of it as in it. They're pretty good swimmers, so you could do a bit in that regard - as well, if they need to go up for air, it can help to have little landing zones so they don't have to do a surface run all in one go (they do it just because - or if someone is feeling a little down in the gills before you catch on).
@arenaboy007, what kind of filter do you use ? I have sponge filters in my tanks and they're really cheap and work great ! If you do change to a bigger tank, run the older filter along with the old one for a couple weeks to get the new tank cycled quickly.

If you do go that route, you could probably sell what filter you have now to recoup the cost a bit.
I've also heard of people running more than one filter full-time - but I have never done this, so I'm not sure how that would work best.
Or use the old filter to start a new tank. When I moved my axolotl to a bigger tank, guppies moved into her old one.