There is no technical difference in the name registration. All domain names are ultimately stored in the central name repositories. So there is no difference in uptime or whatever.
The differences in registrars come down to things like service, services (do they provide their own dns hosting, private registration, etc..) and price.
Service wise, you need to look at their policies in the event you let your domain expire and want to get it back (maybe you forgot to renew it), and how much that costs.
As for transfer, you pay a fee for the transfer. The impact is that it takes some time to perform.