As grjohnson said they are very similar, and you should be able to achieve a passing grade with the 8STS however, if you want to be completely sure you sholud purchase the 8iSTS.

Of course these are only sampler tests providing you have learnt and understand your 8i Material you should have no problem passing with the STS or not.

There are slight differences but 8i releases are mostly 8 with lots of bug fixes, to understand the main differences look at the release notes on technet between each version.

Regards