If you don't sell it, then you'll end up only using one or the other.
The rMBP is faster, so either you'll gravitate to that machine for most of your computing needs or you'll stick with the tried and true of your 17" MBP. In that case you would have wasted money on buying a computer.
I'd say sell the laptop, don't live in the past, enjoy the gorgeous screen the rMBP provides and look forward to the cash the 17" will generate for you