I think your question is already answered in another thread. Anyway Lnb15k is better than most others. You will learn this from experience
If you can get 15k lnb for C band then go for it. Don't expect a big differeance between 17k & 15k. Most of these things written on LNBFs are just under test conditions if at all they are true. Don't worry about them. Lesser the noise figure then go for it. This is what we are also having so don't worry.