black diamond speed 40 vs 50

Picked up a BD Speed 40 pack a few weeks ago and took it on a 5 day climbing trip.  Although it was doable, I found it to be a bit small for a trip of this length so ordered the 50 liter version.  

I had planned to keep the 40 for shorter trips or for warm weather, but I find the weight difference inpercievable and as best I can tell, when everything is cinched down the two packs are the same size when lightly loaded. 

Debating unloading the 40 and just keeping the 50.  Originally I planned to keep both as I usually don't need that much space but for like 2 oz different and a design that cinches down so nice when the extra volume isn't needed, Is there really any reason to have both??

How do the new Speed 40/50s carry weight? Any thoughts on comfort regarding the thinner hip belts and shoulder straps and 30-40lb loads? I have an older version that I'm considering upgrading due to the frame stays failing. 

Only reason is just have a spare pack. A 40 is a pretty good size for sport cragging and won't wear out for bigger trips where you need the 50, otherwise just unload it. 

I had a 40 that I took on a 4 day ski tour on on Shasta carried around 40 lbs. carries fine although much more may have been uncomfortable. 

Ended up exchanging for a 50 since it is a fair bit bigger and only a couple oz heavier. 

Are the back panels the same height, e.g. it's just a difference in circumference?

I use the speed 40 on 2-3 day routes because it forces me to pack light. If we didn't have some much bushwhacking BS in the PNW I'd use a speed 30 and have more outside the pack, that size compresses down more. 

Will you be able to keep yourself from throwing in extra crap with the 50l pack on your weekend climbs?