Do you think clients choose your service because your competition doesn't have an uptime guarantee?
Of course. There are so many providers in this industry, many offering the same type of service. Would you rather purchase from a company that has a guarantee (and offers credit if it goes below that guarantee), or one that doesn't? Of course you'll go with the one offering an uptime guarantee.
If you do offer a guarantee, make sure you can fulfill it.
Well, I don’t' see why you shouldn't offer it. It's suppose to make your clients feel more comfortable when signing up with you. Its also a great marketing technique on your website.
Our company offers a 99.99% uptime and if its not met in a given month, the client will get that months of service for FREE. Though we never had encountered that much downtime yet, I always intend to live up to it.
The only big exclusion you want to add is that the uptime guarantee does not go against the "service maintenance or server upgrades etc..". Of course, we do some service maintenance and maybe a reboot or 2 a month.. This shouldn't account against you.