To the OP, I don't think you are wrong. Gilbert seems to be a little slow footed due to his height when it comes to marking WR's, if thats the case at the next level (except he is perfect for playing big WR's) then you could probalby protect him at FS.
But though he might get beat deep occassionally in the NFL, his value at Corner might be too much for us to pass up. I don't know how either him or Tillman would project to nickelback,
but I tell you that Gilbert on the outside with Tillman filling in FS and Hayden at Nickel with Jennings our 1A would really look good. Giblert can follow the big Wr's and let Jennings do his thing.
Adding a playmaker to our secondary would give us alot of help we need, and I think in the end Tillman's versatility will allow us to make the most out of the best pick for our future.
I still would welcome Pryor or Dix with laughing open joy and thanks, but Gilbert does about the same for me as long as Tillman is on the field somewhere and we get our top 5 secondary on the field often.