The 58 is only 3oz more than the 46, so I would go for the 58. I have a 46 that I like a lot, but I have that size because I got a super deal on it. I would get the 58 in a heartbeat. The BPL framed pack state of the market report noted that the 46 seemed small for the rated capacity.
The Exos put me off frameless packs. Having that frame delivers excellent weight transfer and the ventilated "trampoline" back panel means no gear poking me. Packing is done with consideration to balance— you can load things wherever you want. The extra weight of the pack is a trade off for the comfort it delivers.
More to the point, the Exos packs have excellent compression features, so you won't have any problem making it smaller. You can run the lower part of the side compression straps *under* the side pockets too, which I love— no interference with getting stuff out of the pockets and the stuff inside is compressed away from the pockets, helping more. There are loops installed on the sides of the outer panel above the big stretch pocket if you want to add bungee cord and compress that area too. I love that big stretch pocket for my sit pad, rain gear and maps. As others noted, you can remove the top pocket as needed.
BTW, if you ever break the waist belt buckle in the field or need a little more room, a carabiner works great.
NB: IMHO, the weakest point on the Exos is the bottom corners– if you throw it down on rocks and hard ground, they will fray over time– I've seen that on old ones. I'm going to split some clear vinyl hose and make some snap on scuff guards for mine.