You should write your prototype using whichever framework you’re most comfortable with, which may be no framework at all. Make sure that the relevant bits are separated out as much as possible, so that it won’t be too terrible to move to something more powerful later.

A prototype needs to get done and thrown away as fast as possible. If a framework doesn’t serve that end, then don’t use it.

I would never prototype anything in symfony; it’s very heavy and has big start-up costs, to say nothing of the learning curve if you’ve never used it before.

People sing the praises of prototyping in Rails but I’ve never got around to learning it properly, so there would be another big startup cost there.