If the "I-99" you are referring to is Rt. 15 I can tell you this: From the PA/NY line above Mansfield south to Williamsport it is all 4 lane and bypasses all the small towns that "old Rt. 15" used to go through. Nice drive; only one healthy grade that I can think of at the moment and that is as you approach Steam Valley. It is designated as the "future" I-99 corridor.
South of Williamsport Rt. 15 is a mixture of 4 lane and 2 lane and does go through some urban areas and small towns until it intersects with I-80. Also a nice drive but don't expect to make good time.
I-81 sees heavy use and thus a lot of construction. Currently there is construction in the interchange area with Rt. 309 at MacAdoo.
Would help to know where you are headed.