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i'm not sure if anyone can help you here. it is something you need to test and see if it fits your business. try both models for 2 weeks each and see which one has a better correlation to actual conversions. Spend X amount for 2 weeks, Model 1 = 100 conversions Model 2 = 90 conversions now plot a graph of daily spends vs conversions and look at correlation line.

the two people i was hoping to get a response responded. i was not even sure how "time decay attribution" model worked for sure.

time decay is simply a fancy way to say that the model assigns most importance to first touch, then slightly less to second touch, even lesser to third touch and so on. whereas last touch is 100% on the last touchpoint.