Both platforms have same function. so I see no real big difference between both. They are both automatic upvoting platforms for Steemit. For instance, you have a favourite Steemian whose work you're interested in, you can easily place them for automatic upvote from you using either steemvoter or steemauto. There are just slight differences, although perform same function. Some of the differences are:
Login system: For Steemvoter, you have to create a new account with them using your email, creating a password and all of that procedure, but for SteemAuto, you just simply login with Steemconnect once you have a Steemit account.
Limit: SteemVoter has a limit on the number of Steemians you can place on your automatic upvote list. On the other hand, SteemAuto is limitless. There was a time I got a decent amount of DSP and decided to place a couple of persons for the period of time it would last, with SteemAuto, I was able to add up to 50 Steemians and could still add more with time as there were no limits to the number of persons you can add via SteemAuto.
Fee: SteemVoter charges a little fee in form of Steem/SBD monthly for their premium account whereas, SteemAuto's services are free.
Those are the little differences I can point to, otherwise, they both offer similar services.
