have you ever tried any of them before? i had never even heard of this Propanolol before, but after a quick google search it seems that it isn't exactly for anxiety, more like hypertension. try both and see what works better for you i guess.
Alprazolam is gonna be like an almost instant anxiety relief. if you have to be anxiety free for a longer period of time though (like 6 hours for example) i'd rather go with Clonazepam as it lasts longer.
just don't do them every day and don't abuse them. imo, benzos are GREAT for a short relief from anxiety, when you really need it, but only for that, a short relief. they aren't the cure. if you have overall anxiety problems benzos are definitely not the answer, go with CBT and therapy man...
again, don't use them often and don't abuse them or you'll get addicted. i never got addicted but everyone says benzos are a terrible habit to kick... i probably have not stressed this last part enough... but you can see around here lots of people talking about how addicting benzos are and all the likes... so take care with them...