I have a young star CF who is going to arb for the 1st time and he will cost 1.7 or I can get a 5 yr 7.5 per season contract which runs out at age 29.

At 1st glance it looks like the arb is the way to go but his patience is almost nothing so whats the downside. I can't lose him he is the corner stone of the rebuild franchise.

What do you guys think?

Arb.  You will get another chance to sign him to the big contract next season and the season after.  At his age you want to control him for as long as possible.  DO NOT actually arbitrate him 3 times; with that low patience he is very likely to leave you without negotiating if you do that.