Do you mean either/or? Or both? Because both would obviously be the best, I wouldn't think the catless midpipes would be as useful when the rest of the exhaust is still more restrictive, so if you were only going to do one I'd choose the catback. But someone more educated than I may think differently.
IMO, The STD rear mufflers are first point of restriction and the primary cats are the next. The 2nd cats and remaining exhaust system are not likely to be too restrictive unless you're chasing big power ( turbo upgrades etc ).
If you want a restriction free ( but legal ) exhaust, you'd go for 400CPI primary cats with 2.5" pipes and 2nd cat free downpipes and then a 2.5" system BUT, two things will happen, you'll get over-boost issues ( and need a full ground up dyno tune ) AND, the drone will drive you Nucking Futs !
Honestly, so long as the rear mufflers flow well, I wouldn't bother with anything else.
I'd go with catback --- stock has these crimps near the end (look up pics to see what i mean) that narrow the pipe to like an inch or less presumably to reduce noise/resonance
I did the catless secondary downpipes as well (initially before catback) but ended up removing those and going back to stock
no dyno numbers or anything but did feel like the secondary downpipe (or catless mids) didn't really do much on the butt dyno except make more noise (deeper/louder) and make the 1-2k rpm range annoying for my tastes (reverb). It did however bring out some mellow burbles which i liked when letting off the gas
do miss the burbles now that i have only catback and stock downpipes/midpipes.. but what can you do
i felt like 1-2k range reverb was annoying for my (i'm oldish ~37) ears on long highway road trips so to me it was worth it to revert that back to stock even tho it did mean no more burbles =(
find out what is important to you! i think you have enough info here to make the decision knowing what to expect
As has been alluded to, unless you are going to add a JB4 or some other tune there's basically zero performance gain by replacing components of the exhaust system. Doing these things is more about sound than anything. This is why every exhaust video ever recorded is attempting to highlight how it sounds, not how it "performs".
Even if there's claim for X more horsepower, you're never going to feel that in daily driving, ever.
If stock is a 2 on the loudness scale at wide open throttle, then upgrading the mufflers (most commonly done via entire catback replacement) brings you to a 3.5, replacing the midpipes with catless ones bring you to a 4. For reference a 10 here would be open header muscle car. 500 would be a space shuttle launch.