both cost 0 mana.
Blade Mage is used in aggro/tempo shells as finishers directly after playing Daria. You'd be surprised how often late-game Blade Mages just straight up win the game the turn they're played.
Flame Destroyers on the other hand, while having far higher stats, doesn't do anything the turn you play it, which makes it unsuited for aggro shells and more suited for D-Shift or more midrange builds.
Potential to become much bigger is not as powerful as having large base stats or a powerful storm effect. Summoner can get huge, yes, but under certain conditions. If you draw into it late game it's a 1/1 for four. If you draw into a Blade Mage it's two direct damage, right then and there. And a Flame destroyer is a 7/7 body. Tempo Runecraft aims to do a ton of damage early so late game they only need one to two spells or followers to close out. Summoner outside of the opening hand or an early draw is just not as consistent for either the Tempo or D-Shift shell.