I purchased a Pinnacle to have as a ultralight pack for my girlfriend and as a loaner for when we have any other less-prepared female hiking with us. We did a first trip — just an overnighter — recently with my sister carrying the Pinnacle. Believe it or not, we actually made use of all the space in that pack. I shoved a Z-Lite in there as padding and to take up room, and by the time I added a zero degree bag (~4 lbs), her extra clothes, some water and a share of the cooking stuff, it was pretty much full.
I'm in that stage where I have a pretty decent UL setup for me, or for just my girlfriend and I, but I don't ever plan to have enough extra UL gear for other people… but the Pinnacle seems to scale well even as the load size is reduced. And for a lot of people — especially if you are using foam pads — you can quickly fill a lot of room in a pack without actually adding that much weight.
In my limited experience with a Pinnacle, I wouldn't recommend a fully loaded weight (including consumables) of anything more than ~25lbs, however with a foam pad and a synthetic or 600-fill cold-weather bag, it is really easy to fill all that space.
Bottom line: if you ever plan to take the pack four-season camping, I'd totally go with the Pinnacle over the Jam. And if your wife/girlfriend is a cold sleeper and needs a higher-rated bag and a foam pad, then you might need it for 3rd season camping as well.
IMHO the biggest drawback is not having hipbelt pockets — I LOVE the pockets on my Jam. But that's not a make-or-break issue, especially not for less than $60.