The MBP will absolutely smoke it. The G5 will be a long way from keeping up.
This isn't PPC hate, but the machine is now very old and CPU tech has come a long way in the past 7 years - and the MBP 2011 cpu is FAST, even by today's standards.
For many multimedia tasks, the Core i series cpu have specific instructions to speed up such things. They have hardware AES encryption instructions, much more cpu cache, etc.
Obviously there is a massive price difference, but in terms of performance, it isn't even going to be close.
As far as heavy internet usage goes (one of the OP's criteria) - don't expect new versions of flash, video codecs, etc to get any PPC love. The platform was end of sale 6-7 years ago now, so new software will be thin on the ground.
Buy all means, feel free to get the box, just keep the above limitations in mind when considering price.