Unless the tank is very heavy with LR (creating a lot of issues with dead zones), I would choose a MP20 for the wider flow pattern. Since cost is mentioned as being a factor, I would pick the MP20 over 2 MP10s to save $100. If it came down to spending $400 on two MP10s, I'd rather just by a single MP40, again for the wider flow pattern as well as its greater overall functionality with its ability to run in a master-slave configuration -- especially if a tank upgrade is planned down the road. The dry side on the MP20 and MP40 is the same thickness, so external size wouldn't be an issue if there's already room for the MP20.
Moving the tank forward would be a pretty big chore. With a tank of that size, even removing most of the rock and half of the water will still leave a pretty heavy tank to move. If the tank is sitting on medium to high pile carpet, it'll be even harder since it'll have to be lifted up higher to get out of the indentations it's made. I've never tried to move a running tank larger than a 55g, so I don't have much else to offer in that regard.