Here's my 2 cents, I'm currently a C-17 pilot in the reserves while flying for UPS...the C-17 is an awesome jet to fly! it can be tough to stay current if you have a full time job, but you will see the world and rack up alot of hours. No offense to C-5 guys, but run fast from a C-5, I flew 141's on active duty for 7 years, 6 of those flying airdrop and special op's which meant I could have had another thousand hours if I just hauled trash. I left active duty with close to 3700 hours, my C-5 friends had way less than 2000 and tanker friends had even less. If your looking to build quality flight time the -17 is the way to go. Ater I was hired at my airline I knew so much more than even the real senior Captains who flew internationally. It really prepares you for any airline job.
Good Luck