Ride the Pivot M5.7 you've ordered as-is (once you get it) and see. If you have chain drop issues, get one. If you have no chain drop issues, don't get one.
Then, it'll depend on if you are running one or two rings, do you want bash protection or not, and what's your budget. What kind of trails you ride, and how you ride comes into play quite a bit as well.
Asking which is "best" will just get you a ton of answers based on what people like for themselves.
I just went to 1x10 with a WolfTooth Chainring 30T on my trail bike and so far 2 days of riding through some rough terrain it has not even fallen off once. No chain guide, just a rear Shimano XTR Shadow Plus derailleur. So it is possible
My experience with three bikes set up 1x is I dropped chain without a guide. One bike went many rides before dropping the chain, but once is one time too many for me so I got a chainguide.
I used Pauls on two and MRP on one and they both were easy to mount and have worked flawlessly.
I like the idea of a clutch rear D and the wolftooth chainring, and may try that when it comes time to replace a rear D. The price of replacement chainrings is the only drawback I can see to that system.