Bookshelf or Floor standing for ~$900

Looking to get some new speakers and am wondering how my money would be best spent. I will say that I always buy used/demo as I view audio value similar to automobile value. Take it out off the lot or out of the store and its losing a lot of value. 

My room is roughly 14x24ft and the speakers will be along the longer side. 

I've been eyeing some Monitor Audio Silver 100s for $750 as well as some Martin Logan Motion 20s for $950. 

Will two 5in drivers be better than a single 8in? Are there other options you would recommend? more generally (as the title suggests) should I be looking at bookshelf or floor standing speakers in this price range?