I think bill has covered this off pretty well. Wireless Repeating can work well at times, but be sketchy at other times. Potentially you can actually degrade your primary AP wireless coverage, even though you are extending it in some other areas.
I know you don't want to run an Ethernet cable between the AP's. Maybe it's because the cable run is impractical?
Have you considered using Powerline adapters between AP's instead? This should maintain good/stable connectivity between the AP's without degradation of service. Just make sure the AP's are on different non-overlapping channels (1, 6, 11 on 2.4GHz for example).