Heart rate will go up with a lower gear ratio at the same speed, but for an after-gym type ride I'd be more inclined to go with a lower gear ratio and just ride slower. Too high a gear and you'll be using more muscle than you want (well, unless that's really what you want).

Also, I've found that 46/18 is a pretty good "easy" ratio. Though I guess all of this ultimately depends on how fast you actually intend to ride.