Use "I" because it's more personalized. The excuse for using "we" is simply that the service may seem professional; hence, building more trust. That might actually be true, but unless you're building an online bank, it wouldn't matter.

I initially worked on my webservice alone, and decided to adopt "I" on every possible location. I managed to build a dedicated community around the website, and later on decided to use "we" to denote the community as well as myself. Now, when I say "we thank you for participating", I mean "me and the community...".

Of course, if you've more than one founder (like I do now), you should use "we". Be consistent with the terminology in order to avoid confusing the end-user.