Danno, when figuring your arrow size/spine there are a few things to take into consideration.
One of the first things you'll need to do is determine what length your arrows should be. This can vary some due to the individual's preference. I prefer that my hunting arrow sits about 1" in front of the riser. This will keep those precious fingers safe, and a longer arrow is also more stable. Your best bet is to go to your local pro shop and have them check this for you.
Your draw weight will also determine what arrow you should be shooting. I'm assuming that you're shooting a 60-70# bow. If you're positive that you're going to shoot 65lbs and only 65lbs, then size your arrows accordingly. But if you're unsure of your draw weight, I would recommend that you size your arrows up to 70lbs, that way if you happen to increase your poundage, you're not having to purchase new arrows. Also, its better to be a little 'over-spined' than 'underspined'. When your arrows are underspined, they tend to have a more erratic flight pattern. Sure your arrow might hit that buck right in the "sweet spot" but if its angled up or to the side, good luck with getting that pass-through.
Your cam design is also important when sizing arrows. They usually are listed as a Medium Cam(dual or hybrid cam) and Single or Hard Cam.
You will also want to know your field tip/broadhead weight. This isn't a big issue unless your concerned with you FOC.
Also, you had asked about the pros/cons of a heavier arrow - A heavier arrow will usually produce more kinetic energy, which will inturn give you better penetration. But at further distances, a heavier arrow will drop more drastically than a lighter shaft, so yardage is more crucial.
I would suggest stopping by your local pro shop and have them check exactly what size/spine of arrow you should be shooting. It will be quick and easy, and should cost you little to nothing. Hope this helps.
