I see online there are two boat services from Chetumal to San Pedro and back, which is the better and more reliable? Thanx

I looks to me like they run on opposite schedules therefore choose the one that fits your needs.

I have used both. My preference is San Pedro Water Taxi Jet Express (located on the lagoon side).
You might not have a choice if they are still operating on the alternate day service that they were operating on last year. Best check with them to see if they are still doing that.