I have rods in both lengths. My 10 footer is a Greys Platinum XD which weighs less than 4 oz. The 9 1/2 footers are a Sage 6 weight XP and a Sonik 6/7 weight, both of which are lovely rods and weigh about 3 1/2 oz each.
Personally I can cast further with the 91/2 footers than with the 10 footer. However I like the 10 footer for boat fishing, the extra reach is a great help here.
I can fish all day with all the rods, so that side of things does not worry me.
You don't mention the make of the rod you have ordered.
If you experience a sore arm. or have back ache, you need to look very carefully at your casting technique. A good guide is the number of false casts you take before final delivery. You shouldn't need more than 3 false casts; if you do, a session with a casting pro would be a good idea.
For short casting on rivers or small still waters, a couple of false castsare all you need, except when you are false casting to dry your fly.