Both Richard Solo and 3G Juice aren't available before end of August. Solo's offer seems to me a fine choice since the company exists longer then today. The 3G JUICE is new. It's the better battery with 50% more power, OFF-button to save energy for months and a endorsed by the boss of LinkedIn, see the back of the package here:
http://www.3gjuice.com/voorkant.pdf (2 Mb, PDF)
If I go for what others do, I would go for the 3G Juice. That one has a sales rank #2 in Electronics > Accessories & Supplies > Batteries, Chargers & Power Supplies > Battery Chargers > Telephone of Amazon, the one of Richard Solo is not in the top.
3G JUICE (Amazon)
Amazon.com: 3G JUICE iPhone 3G/2G & Ipod Battery Extender - Certified by Apple: Electronics
RICHARD SOLO (own site)
Backup Battery for iPhone / iPod - DX001
See also
Several backup batteries can extend your daily iPhone 3G use