I am currently using my internal soundcard because there is no driver for my studio card yet for Snow Leopard and it performs like a champ. I have used the internal card many times over the years. Apple did a great job with that internal sound card. However, if you have an extra Motu you might like that better for latency etc. I would try both and see the results. For me, I didn't see much difference in performace but I got lots of additional control through the mixer software that my external came with. It made channel routing simplistic compared to internal routing.
Either way they will both work.
Maestro2be