Depending on how you like to ride and what trails you like?
If you are going to ride the Tennyson Trail then generally on the western half between Newport and Freshwater Bay if you go Newport to West Wight then there are, on the whole, longer downhills followed by shorter steeper up hills.
When I used to ride it to and from work/Gfs I would ride the road to newport and the trail home to freshwater.
If you are heading on that trail then a definite stop off would be Brighstone forest. You go through the middle of it and there are some nice (quite hidden) single track lines and some DH and jumps.
Unfortunately I am in cornwall at uni but the Isle Of Wight is my home land 🙂
p.s. if you are going around the island then you will mainly be on the roads?