Hi Dave,
For basic recording/editing/mixing, you're not going to be missing a whole lot.
That's PT's forte'. (Straight up tracking/editing/mixing)
PT is 32Bit... so it's limted to addressing 4GB of RAM.
If you're making extensive use of virtual instruments, that limitation can be tough.
The way around it (for PT users) is to use VE Pro to host soft-synths... running alongside PT.
IMO, It's simpler to just use Sonar and forget about it.
There's nothing you're going to do in a typical session (using PT) that couldn't be done in Sonar. In many cases, because of your experience with Sonar... it would be quicker/easier.
FWIW, I have many Pro level clients... and they use whatever tool/s they want to accomplish specific tasks. These folks are hired for their ability/skills... not for the tools they use. If a potential client is more concerned with your software than the end-result... I'd give that gig a second thought.
As for "compatibility", you can deliver Wav files... which can be loaded into any DAW application (including ProTools). This works 100% of the time...