You will be in the window for prime Baetis hatches. Usually that 2nd half of September is fantastic on both stretches. Caddis as well will be hatching heavy so if you like to swing soft hackles, you should find lots of takers.
Middle Provo will be almost all browns, and lower Provo will be about 60/40 mix browns/rainbows depending on how close you fish to the Deer Creek Dam. Fish run larger/heavier on the Lower Provo (12-15 in average with many larger fish available).
A lot will depend on the weather... an early fall with cold weather will get the browns thinking spawn and they will be very aggressive going into October. Prime streamer fishing.
Both stretches are tailwaters, so three (3) items are on the menu year round; Baetis nymphs, scuds/sow bugs, and midges. Large numbers of black fly larvae are present as well. You may still see some PMDs too depending on location.
A good guide will hook you up. Who are you booked with if you don't mind me asking?
As far as where to stay, hotel in Lehi, Orem, Provo puts you in proximity to the Provo Canyon where the Lower Provo flows... the middle runs almost perpendicular to the lower in the valley beyond Deer Creek Reservoir (below Jordanelle Reservoir).
Can't really go wrong through that time window, weather is typically pretty stable that time of year (~5700 feet ASL) and the flows are steady as well.
