I would choose ghost shrimp, simply because shrimp are prey animals and it's almost inevitable you're lose one if not more eventually. Ghost shrimp are only about 10 cents a piece and RCS can run you $5 a piece typically. Ghosts also have the benefit of added camoflage.
I just got 5 ghost shrimp for my Little Guy, and so far the most he has done is chase the around and stare at their tales. I don't think he'll eat them, he just likes to play cat with them as the mouse :P I'd say go for ghost shrimp first to see how he interacts with them, they are after all the cheapest. If they die, then you sort of have an idea of his personality and can make decisions about tank mates afterwards. Make sure the shrimp have places to chill out too though.
ghost shrimps are great as stated above they are prey and are sold as feeder so they are cheap and possible disposable they only live for a year or 2 unless you figgure out how to unlock thier fry/ shrimpetts survival needs. so if you don't like them you will have time to decide if you want them again.
if you think your fish will eat them then sperate them for a day or 2 in the store bag with an aerator and load them up with peas. thier stomach will digest the peas enough to not bother your bettas gut when he eat them and he will get a good for him snack.
how ever if you have hidey spots you ghosts will survive.
they are good at keeping a tank clean but they are not cleaner shrimps. that means they will eat left over food and fish poop but not so much on the algae that grows. but by eating the poop and other left overs they remove potential water pollutants before they pollute thus helping keep a clean tank clean. they present no load on your bio filter or mechanical or chemical filter and don't really count against your stocking quota for the tank. in fact they allow for you to keep some messier eaters in a little less water but don't go over board stocking your fish.