Dermott's usage is ridiculously limited compared to Gardiner - making comparisons is way too difficult with the sample sizes and information we have available to us at this point.
Dermott looks great so far, and you hope he can step into Gardiner's current role within the next year or two, but it could be a big mistake to bank on that before he's actually ready.
Re-sign Gardiner, move on from Brownie in 19/20 if the cap doesn't move much, and look to trade Gardiner only if you know you can comfortably replace him internally, or if you're getting his replacement back in the deal. Just because we re-sign him doesn't mean we can't look to move him later.