Any question answered...

Which is a better thermometer, oral or rectal?

Depends on what you're into.

Rectal is more accurate, temperature-wise, though. I don't know if you'd find it more or less fun.

Used correctly they both should be accurate.
Rectal for people/things that may bite down or be difficult (babies, animals dangerous offenders) and oral for people who follow orders and don't want things shoved up their arse. If you fit in to neither group, you might as well flip a coin.

Anyone used a Top Peak Tribag before?
Are they worth getting to store a few bars/jelly babies etc?

Yes, if that's what you want then yes. Waterproof one is pointless (since most food is wrapped in some way, or at least can be), after looking around I think wheelies were the cheapest, with a Raleigh version at £7 iirc?