that is a really really tough choice. both are great instruments. i play copelands, high and low D and C. i have met Michael Copeland and visited his shop in philly. great guy. i’ve played abells before; and I’ve also met Chris Abell at his shop a few years (as my sister moved to Asheville, NC.) Another great guy. but i’m not as intimately familiar with abells as I am with copelands. so take this with a grain of salt.
A Copeland D will give you a strong, aggressive chiffie sound. lots of room for bending notes and articulation. they have a downside of being very easy to overblow, esp in getting shriekie in the upper register. Since copelands can take alot of air, i think many people underestimate the importance of breath control with them.
An Abell will give you more mellow and sweet sound (it’s made blackwood afterall, not brass). it’ll be much much better in tune overall than the copeland across the low and high register. I’d say they are easier to control.
For what it’s worth, here’s a tune I recently recorded on my C copeland that highlights the kind of delicious bite copelands can give:
http://brendanjanis.com/misc/ophelias-ghost.mp3
Sorry, wish i could do equal justice for the Abell whistle but I don’t have one (althoug I wish I did). You’ll be happy either way you go. Good luck!