I've tried both. The one from http://www.realsentrygun.com/ is a lot better IMO. It's more popular and has been around a lot longer. It tracks better. The accuracy is much better. The target filtering is much better. It's a lot less buggy too. Their system has been used by Valve for their portal turret, by The Discovery Channel, and by channel 5 in the UK. This isn't an arduino vs pololu thing, it's just that the sentry project's software is a lot more developed. It's likely because they've been at it a lot longer.