I use 3-4 oz quite often for straps as that seems to be the most common weight for all parts of cowhide pouches from the late 18th - early 19th Century. 4-5 oz on the other hand is not unknown and is a bit firmer than the lighter weight. This is assuming the strips you are using are cow and not deer since deer is generally quite a bit softer than cow even in bark tan.
If your selling see what your customers prefer.
BTW - if you're going to do a lot of pouches I'd recommend buying side or shoulders in the right weights and get a strap cutter to save money in the long run. The wooden strap cutters from Tandy, etc. are relatively inexpensive and work just fine - just keep sharp blades in it or them (when I was running my leather business full-time I had several strap cutters pre-set for the most common widths such as 3/4", 1", 1 1/2", etc.