Yes, an R value of 5.7 will be much warmer than 3.2. (I assume you're looking at an XTherm vs the XLite?) I sleep a lot warmer on my XTherm.

How long is your trip? What's the coldest you've slept in these 40 degree bags? Are they honestly good down to 40 degrees? I get cold in my "20 degree" bag well before it gets down to 20 degrees. I wouldn't want to be worried about freezing at night. I've had some long, cold nights before and they're not fun.