GPS options

Right I have been thinking about this for a while, what do I do.
I have a garmin nuvi which I have had for a while, just managed to convert my euromaps to mac,
I also have an etrex with gb topo, my question is this do I get a quest for the bike, use the etrex, with navigator loaded on, can I use the euro v9 maps I have already just by loading them on a sd card.
Etrex buttons could be fiddly to use, quests cheap at mo.
or do I spend lots more dosh on new zumo 660 or rider 2

HELP

Which nüvi do you have and have you tried using it on the bike? I used SWMBO's nüvi 770 on a rented R1200GS in California quite successfully. Ok so they're not waterproof but there are ways around that. I liked it so much I bought a 760 for use in the UK and I'm planning to mount encase it and its RAM mount in a polythene food container to waterproof it.