The 500 has a display, the 930 does not. The latter supposedly has a faster ASIC, and also different radios, but for now both have the same functionality according to Megaman support.
In all honesty I wouldn't be investing in a Link right now if it's not central to your project. Within the next few months (read probably more than a year, long story, too long to go into here!) there will be another Link that is anticipated to support Apple HomeKit amongst many other technologies. IFTTT is already supported (via LightwaveRF's cloud based services), and there's more to follow, but it's expected that a new Link will be necessary, just as Samsung have their SmartThings hub.