On one hand, we have mRNA, which is self-limiting and that we know exactly what it encodes, as every single base has been put in there by us. We know what protein it encodes. And if that protein turns out to be harmful long term, guess what, the virus has it too.
On the other hand, we have a viral vector vaccine, which injects a live adenovirus which then delivers the instructions? I know we have chosen a harmless virus, but it's an entire virus nonetheless.
mRNA vaccines are seeing their first large scale deployment now, but they are based on decades of research. Humans know a lot about mRNA, which is how they even became possible.
I prefer the more precise approach mRNA vaccines provide. Mind you, they are both good for our current situation. I would have gladly taken J&J if that was the option. Ended up getting Pfizer due to the current halt and I'm very happy about that.
It's just that mRNA vaccines open up entire new treatment options. We have come a long way from having to grow viruses in chicken eggs (which is something that's still done).