Well... here's my humble opinion- Jethro reminds me of the local chain of specialty BBQ restaurants here in central IA. Then I thought Gibbs! (Jethro Gibbs from CBS' NCIS.) Which made me smile. then I thought "Hillbillies."
So... it doesn't leave me with a Hillbilly feeling right off, it actually made me smile. I think it works with Levi.
But it is not a name that Would be used up North.
Ha ha- I love NCIS :)
It definitely feels country and low-brow. I think that it has the option of Jet. I'd avoid it if you surname is anything like Updike.
As a huge Leroy Jethro Gibbs fan, my first reaction was to swoon! Jethro and Levi are cute together and the nn Jet is darned cool!
so, Simon or Jethro? If you are brave enough for Jethro (I like Jethro Simon) then go for it. If not, cause I am sure you'll hear something about the choice of Jethro, Simon Phillip is a beautiful, strong name.