In my opinion, the First Watt is a more detail amplifier being that it is Pure Class A. However, and a big however is that it is very low powered, especially the F7. The F6 is the most powerful FW. The limitations are pretty much in that they cannot drive the systems extremely loud and hard to drive speakers they may not be able to get to acceptable levels. This is not an issue for me because the F6 gets my hard to drive speakers plenty loud enough for me. Many people would require more power.
I have been considering a larger amplifier, such as possibly a larger ARC, or PrimaLuna or possible the Job mono blocks mainly for the possibility of going to a different speaker down the road. Something like a KEF Reference for example.
But for me, the JOB 225 was great, and had a lot of power. The only real draw back was that it is very high gain so something like the Rothwell Attenuators was a requirement to achieve any kind of volume control from my Pre-amplifier. In absolute purity I do prefer the First Watt, but the JOB if very good and very acceptable. So if more power is needed, they can't be beat!