So this is an opinion question. Means no one is wrong or right, really.
I believe the easier tool for performing a host of calculations might just be Excel because that's what it does best. I don't know what is meant by them being not user friendly. Should be like anything else, I guess - easy if you know how or what to do. There is a lot of power in presentation by using merged columns/rows, calculated cells, conditional formatting, lookups or whatever. Workbooks are also fairly powerful from a vba perspective, and are hands down the winner if you need charts. I do agree that Access is better for storing/retrieving data as records and creating professional looking reports, but I still think those reports are best suited for dealing with a multitude of records rather than a one-off situation like an invoice for a small business. I've used Access for many years, but have and would still lean towards Excel for invoicing. The best solution might be a marriage of both applications.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.