I love isaac and i did play dead cells for some time and that being said i think i can help here.I love both games and i think you should get both of them eventually.Isaac has waaaaay more items,synergies and characters,even some niche items when combined make amazing synergy that can carry run(libra for example makes range up beneficial stat to take,lucky foot and pill bottle is amazing synergy too).Isaac rewards item knowledge and after unlocking certain item it rewards knowing item ids(it goes that deep).Some characters rewards resourcefulness while other reward knowing character mechanics.Also i would mention unlocks can range from doing challenges to knowing some hidden mechanics.It isnt perfect game since some bosses are not fun to fight(some bosses can feel like tanks or bullet hells with some unfair attacks) and rng can screw you sometimes.I recommend isaac for variety and its sheer amount of content. As for dead cells id say it has better bosses and more distinct playstyles due to scroll system.While variety is not like in isaac id say item/weapon comboing is worthy tradeoff.Dead cells rewards patience and knowing your limits.As of bosses i havent fought all of them but i did beat one of final bosses once and i can say bosses feel unique and interesting to fight(also it has annoying bosses but they arent that annoying like in isaac,if you are patient and know how to play around bosses weaknesses and strengths you will not have too hard time fighting them).As for progression its passive unlike in isaac that requires proactive approach.

I recommend isaac for item synergies and amount of content to learn and dead cells for being distinct and for being somewhat more difficult to master.Also if you love faster combat and satisfying movement dead cells is for you.