You could use either mix as long as it doesn't stop the drainage causing perched water in your container. I have used mulch and it worked fine. I'm going to do that again this year and I'm going to use pine bark fines.. Nothing with dye..

They should do fine with what ever you choose. Does the bottom of the box have drainage? I keep mine in the containers with drain holes and that makes it easier to pull out to store for the winter. I'll just trim off those roots that come out of the holes....

Take care..

Laura

Always use a well draining mix in your containers..

Any concerns with total weight of four 40 gal containers on the structure of the wooden deck?

If you use coarse mulch under and around the sides of the 5 gallon pots, your smaller containers will behave as a conventional container would. IOW, if the soil in the 5 gallon containers would normally support perched water, plunging them into coarse material offers no advantage insofar as added drainage or the ht of the perched water table (PWT) is concerned. The PWT will remain something you'll have to deal with in the 5 gal pots.
If, however, you use a finer soil in the bottom of the 40 gallon planters (like MG) so there is a soil bridge between the soil in the 5 gallon pots and the soil in the planter, the fine soil in the planter will serve as a giant wick and pull all the perched water from the soil in the 5 gallon pots - even if the soil in the planter is fully saturated; this, because you will have recreated the pot-in-pot technique employed by many nursery ops. You can still use coarse mulch around the sides of the 5 gallon pots for stability.
The advantage in using the finer soil in the bottom of the planter lies in the fact that you can use a more water retentive soil with that set-up, as opposed to what would normally be advisable, than you could if you used coarse mulch in the bottom.
Here's the reason. Let's say the soil in your 5 gallon pots and the soil you use in your planters both hold 4" of perched water. If you fill the planter with 4" of MG soil, and set the 5 gallon pots on top of the soil in the planter, the 4" of perched water in the 5 gallon pots will move out of the drain hole and take up residence in the soil at the bottom of the planter (the wick), leaving the soil in the 5 gallon pots well\-aerated and free of all perched water.
Al

Al, thanks for explaining how that works, so soil in bottom much on sides