I'd do a 21700 mod since 30Ts are available now...but I wouldn't do 20700 (Mech) again with the ijoy 5 legs. Just my experience with them is why I base my judgement.
I built an Orca 2x700 with Silver internals, and my Molly with Copper and the Molly with my 5As hit harder (DMM at the terminal posts) using the same RSA as reference...Constantly hitting fresh at about 3.6v'ish where my 5As were always hitting 3.79v+ Fresh...
I am sure there are better 20700's than the ijoy 5 legs, but it was my first impression with 20700's and for me it fell a tad flat...wasn't worth the investment nor chase for better batteries to mimic what I already have (had) with my 5A's.
So I sold the Orca, kept the batteries ( maybe down the line I'll get a regulated that will utilize them *shrug*)
Now, again I will reiterate... I'd for sure get 30T's, especially in a mech...but I am not sure if I want to chase a mod that will hit the things I need/want in a mech...Silver
and be a squonk ($18 a battery, would need at least 6 batteries...New mod )
Now with the 6A's coming down the pipeline with more mAh and better performance than the 5A's and still in 18650 form factor...to me it will be a hard sell to invest in 2x700's again.
Just my humble opinion.
Side note: I do appreciate mod makers, China and or Artisan having both 18650-21700 usable in their mods...just haven't come across one that looks good to me.