Was gifted some old HP's today... questions

[tppytel]

I'm a HS math teacher in the States with an interest in old calculating devices. Most of my colleagues are too young to recognize anything older than a TI-30 or TI-83, much less a slide rule. I was puttering around with my 50g at school the other day and showed it to one older colleague - not a hardcore math/engineering guy, but I figured he might be interested nonetheless. He's like... "Yeah, I've got a box of old calculators like that a friend gave me. I'll bring those in for you." Um... sure, OK!

He shows up today with a bag full of calculators and manuals, all in near-pristine condition...

45 + Manual (no charger)
10C + User Manual
15C + User Manual + Advanced Functions Handbook
28C + User Manual + Advanced Functions Handbook + 4 books of examples
48S (dead) + 2 vol User Manual
A few random old TI's and less interesting things

So... score? I told him some of this was fairly valuable, but he was just trying to clean out his closets and keep the stuff out of landfills. The 45 is going to live in my classroom with some of my other display objects for its vintage charm. The 28C I wasn't particularly interested in myself but a quick eBay search indicated that it should be worth something with all those extra manuals included, so I gave those back and told him to eBay them. If anyone here is interested, PM me and I can put you in touch with him.

I've been wanting a small, handy desk calculator for quick calculations that's not just a TI-30, and I think the 10C or 15C will be great there. I generally use my Prime for serious work, but sometimes that's upstairs in my briefcase and I don't feel like fetching it. Clearly the 15C is more powerful but the 10C already does everything I would need for quick calculations. Are there are any other important usability considerations between the 10C and 15C? I'm thinking of selling the 15C and getting a Swiss Micros 15 or 42 clone just to play with and to have more interesting calculators to share with kids.

I could also use a fresh case for the 10C/15C. The bag of HP goodness came with one slipcase that seems original (my stepdad had a 12C with the same case) but the lining is separating from the shell and the shell is deteriorating and leaving dust inside the case. I was looking at this leather slipcase here. Any other suggestions?