Both work awesome if you need help regarding this u can post we are here to help you out 
i dont have this issue on my RB. I have
RB750G running pppoe-client
RB750 running 3 pppoe-client with pcc
RouterOS on x86 PC running 2 pppoe-client
on all those configuration i have the maximum throughput. And also i could monitor the pppoe session directly on my mikrotik.
I’m using an RB751U-2 as pppoe client, using Linksys Wag200 as ADSL modem in bridge mode.
Throughput is at the same level as using the modem directly, I’m extremely satisfied with that.
I would use the modem in bridge mode with a pppoe-client interface on the routerboard.
Perhaps you could tell us some more about your configuration to try and pinpoint your performance issues?
I am aware of installations with FTTC with speeds of approx. 50mbit/sec through a RB751 in a similar configuration.