The first sentence suffers from a "dangling modifier", which is frowned upon in formal English. In that case, you should choose the second sentence.
You can reword the first sentence though: "My objective as a research assistant was to..."
However, for good writing practices, people prefer to see what you have actually done and not what you were assigned to do.
When you say "my objective was," you're not actually saying what you did.
When you say "I aimed," you are stating your actual actions.
On top of that, I'd get rid of "aimed" entirely, which is in essence a synonym for "tried." Instead of saying what you tried to do, say what you actually accomplished. This is a big mistake people make when making resumes and personal statements.