I don't have a Collings ukulele, but here are my thoughts on pickups for ukuleles:
I would strongly recommend a K&K Big Shot Internal or a McIntyre Feather (both SBT's). While I don't like sound board transducers in guitars as I think there's too many overtones etc that just make them sound junky IMO. I think a SBT in an ukulele works great and sounds very natural.
K&K also makes a Twin Spot for ukuleles but I haven't tried one yet. I have one installed in my new Wixom Tenor which I should get in a couple of weeks so I'll review the ukulele and the pickup then.
FWIW, I don't like under saddle transducers in ukuleles, I find them to sound very "electric", that said, as always, YMMV.
Jimmy