i am way stoked with slingshot too. i have tried 2 sizes in both types and found that i liked the 142 recoil best personally...However, every time i demoed was behind a new boat and new wake...sooo... but i feel comfortable with my choice nonetheless.
try both if you can. might even consider trying the next size up (unless you already are trying the 145) especially because how light the boards are.
but if you can't try the other board then the safest route is the one your friend has of course.
if you like the slingshot boots and want to take full advantage of every inch of flex then you could get an '07 for cheaper. that is the route i took. but remember only slingshot boots fit the '07 boards
thought to self: i wonder how much lighter the '07s are without the additional traditional inserts, probably a insignificant difference?