This is really an apples Vs oranges type of comparison. They serve two different purposes although they can be complimentay in use. As tactical pieces in a monohull race they can be really useful but I think less so for Hobie racing.
My opinion: The GPS would be the least useful for competitive multihull sailing except to see your actual speed over ground. The compass on the other hand can be used to see if you are gaining on a competitor or not, set your layline etc. ALTHOUGH it is a lot easier said than done. On a monohull at a few knots you have time to make judgements, on a Hobie at many knots it is much more difficult.
Whatever - if you just need an excuse to buy stuff you certainly have my blessing and I am sure the blessing of almost everyone else here
If you have a better half let her know you NEED the stuff to go faster