Id never go less than a T10. You loose Hight inside, the outside measurement is 12" . So figure interior height to be around 10 1/2 inches on a T8 and Glass slider opening only 8" ( hard to get anything in & out cleaning) versus a T10 interior is 14" and Glass slider entrance 10". You would be surprised what a difference it is from 8" to 10" and it's worth every bit of $50 more. As far as the heating yes you can go with a RHP and a UTH incase the Temp is 65-68 during winter.. Otherwise the RHP is fine alone for 68+ homes/Rooms.
As far as the Hot side temps now I know its probably been mentioned 50 times but make sure to check the Glass above the UTH not the substrate. All my heating elementa match what the Tstat & thermistate says after 24 hrs of things absorbing the heat. Usually about 1° off.
Example.. My Tstat says 91... The hot side probe thermometer says 90.7... If I then check with a Lazer it says 89.3..88.7 and 90.1 in three different places. This is all after leaving the doors closed etc for 12hrs as everytime i clean it/open it a several times during winter it takes hrs to regulate again.