Re: First build questions on what type bike to buy...
Both are good bikes. I've not built on a Felt frame but I hope to some day. I do have a bike build on an Electra Coaster 7, which is the same frame geometry as the cruiser, only using an aluminum frame over the cruiser's steel frame.
With that frame, and possibly with the felt too, you will have to find ways to make the motor fit. There are several solutions in this forum for the Electra frame that I've seen.
The major drawback you will have in a single speed cruiser is the lack of proper brakes. Single speed cruisers generally only have rear coaster brakes which are NOT sufficient for stopping a motorized bicycle. There are options to convert single speeds to use front and rear v-brakes, however and I suggest you look those up and apply them to your build. They'll wear the paint off the edge of your rim eventually but you will be able to stop when you want, which to me at least, is a big plus.