I had both of those bags when my boys were little. The Little Tripper is quite small for a diaper bag. It's good for quick trips out, and would probably be adequate while your baby is small.
However, when the baby gets older and you have to start carting food and bibs or are going to be gone for longer periods you'll need a bigger bag (at least I did).
Also consider whether you're planning on having more babies in the next few years--the Little Tripper definitely gets too small when you have to cart stuff around for 2 little ones.
If I had it to do over, I'd want the Little Tripper and the backpack version.