Hi,
I have been looking at both the WM Summerlite and the FF Merlin. I have a WM Apache Super Dryloft and a FF Widgeon both of which have kept me warm at temperatures far below their ratings. In terms of quality, durability, cut, etc, I find the two brands to be very similar.
As far as your second question, I would say it depends upon your objective. Doing Rainier right now with a 30 degree bag might leave you a bit chilly but would be fine at lower elevations. For later spring/summer/early fall a 30 degree bag would be ideal.