Never accept a counter offer.
The counter offer isn't a recognition of value, it's a reaction to losing what they were undervaluing and a knee jerk reaction at that. The counter offer wont suddenly secure your loyalty again, but it will buy them time to replace you.
Whatever led you to look elsewhere (and secure the offer from Deloitte) will still be there if you accept the counter offer.
Throw in an unsubstantiated promise (promotion at some point, change of role, or location) - all something which can be looked into, discussed etc. If they can't guarantee it, they are enhancing the counter offer with empty promises.
The bonds of trust are now broken in your current firm whatever you do. You are now the one who was interviewing elsewhere. From now on every dentist, doctor or childcare appointment you have will be interpreted with suspicion. You can't be in the inner circle now, in case you learn things which you will take with you. And you will be leaving - they're already talking about replacing you because they are already deciding how to function seamlessly without you.
Your ship has sailed out of harbour now - it's much further to swim back to shore than you think.
Take the Deloitte offer, and besides, life is too short not to dive in to a new adventure with a 20% salary increase.