Unless you do absolutely no domestic flying - I would go with the $500 voucher since 22,500 miles won't even get you a domestic ticket. Plus you will earn miles on the ticket bought with the $500 voucher. I don't really recommend the 5 - 500-mile ugrade certs but they are good for any combination of flights (domestic only) up to 2,500 miles according to their mileage stats - its not relevant that the usual upgrade requires 15,000 miles.

Also, note that United expires all miles in an account if there is no activity (either adds or awards) in 18 months - this is common to all airlines with the exception of Continental and the "old" Northwest.

FWIW, what you received is called a SkyKit which is United's standard package for when there are problems in flight. They have been cutting back on giving them out but rarely will they go beyond the SkyKit even for United 1Ks (top published tier on United with minimum of 100,000 flown miles in one calendar year) - I have a good friend who seems to get these pretty regularly.