The performance gap wouldn't be huge. You'd mostly appreciate the extra screen real estate and slightly nicer keyboard. You might also consider the non-Touch Bar MBP.
I am actually doing that... I was looking at these three models
13-inch MacBook Pro - Space Gray
Ships: 5-7 business days
Hardware
Touch Bar and Touch ID
2.9GHz dual-core Intel Core i5 processor, Turbo Boost up to 3.3GHz
16GB 2133MHz memory 256GB PCIe-based SSD
13-inch MacBook Pro - Space Gray
Ships: 3-5 business days
Hardware
2.0GHz dual-core Intel Core i5 processor, Turbo Boost up to 3.1GHz
16GB 1866MHz memory 256GB PCIe-based SSD
12-inch MacBook 256GB - Space Gray
Ships: 7-10 business days
Hardware
1.3GHz dual-core Intel Core m7, Turbo Boost up to 3.1GHz
8GB 1866MHz LPDDR3 SDRAM
256GB PCIe-based onboard flash storage
But I figured either go "ultra portable" or go "big" with the new touch bar thing. (Might as well buy the 2.7GHz dual-core Intel Core i5 rMBP with all the ports if I'm gonna go with the non-touch bar new rMBP, but I guess this would miss out on the new keyboard and the new trackpad...)
And I was also looking at a 12.9-inch iPad Pro Wi-Fi 256GB but it doesn't appear to allow direct USB file transfers. If it had that I would just buy one of these...