Summer shop question

I'd say exp cards, if you have any servants you need to level up. The CE is alright, but definitely not the best outside of the event.

If you don't need the mats then spend it on the exp cards. Yes guts is awesome but evade on demand from skills trumps it. And even more so, damage CEs trump it as a dead boss can't push you down to where you'd need guts. About the only cases where guts is significant to me are irisviel's np as it's teamwide (although it isn't as useful as say David's AOE evade) and Herc's bond CE which is just amazing in that it provides 3 guts rather than just 1. This is just my opinion though. If you like zombie teams and want guts on multiple units as often as possible have at it and buy the CE (to be fair I bought all 5 copies to help farm and clear the shop so I can't really talk)

Personally I would buy the CE just cause it’s limited and you won’t be able to get it again for some time while you can always farm exp cards whenever

If you already have the other 4 copies of the CE, then I'd probably buy the last copy to limit break it. If not, then I'd just get the exp cards. Though take my opinion with a grain of salt, because I'm a pretty new player and probably don't know what I'm talking about.