Hello; In my experience the snails do a better job. That said, in my experience neither will do a complete job. The glass will need to be manually cleaned by you from time to time.
On the plants it will depend what you are wanting cleaned. Snails do a fair job on some algae, but not so much on the slime algae that forms sheets.
The fish I know as a chinese algae eater tend to become less interested in algae as they grow. They also become pests to the other fish, chasing and even reported to suck slime from other fish.
You are perhaps thinking of a different species of fish that you call a siamese algae eater. If so, then I do not have any experince with them.
good luck