I'd really recommend a backpack carrier for the 5 month old. It won't be long until the baby is too big to comfortably carry in the front, and the backpack will serve you well for a few years. I had a good Kelty front carrier that I used everywhere from hiking to shopping, but around five months was when I switched mine to a backpack for hiking, as soon as they could sit up pretty well. I tucked rolled blankets around them to help support them and to snug the fit in the carrier. As for which backpack, it just needs to fit you well, just like any backpack should fit you well. We have a Kelty which fits my husband perfectly, but it won't adjust small enough to be comfortable for me. The pack in my profile pic is a Yakima (no longer made unfortunately) that fits my narrower shoulders/hips really well. If I had lots of money to burn on new carriers, my dream kid carrier would be this:
Deuter Kid Comfort III Child Carrier - Free Shipping at REI.com
Just be sure to get one with a frame if you're planning to use it into the toddler years. The frameless carriers just aren't comfortable with very much weight on board.
