It all comes down to YOUR anticipated volume. Take your (realistic) gear and food for the longest trip you expect this pack to carry you. Do stuff your sleeping bag in the bottom of the pack instead of in a stuff sack. Try the 34 first and work your way up.
I've scanned my gear list to give you an idea of what I can squeeze in for a week (it's a pretty shoddy scan, but legible). YMMV, especially with food choices so bring your typical food along to pack in.
Who needs the 58? Winter hikers, family hikers carrying for others, hikers with large volume gear, long distance hikers with minimal re-supply. If you are in those groups buy accordingly.
Let us know how you go and good luck.