I have a 4i (22 Deg) & 2i (18 Deg) - which 20 Deg club to get (7W or 5W)

You really can't go just by degrees. Club dynamics are different between fairways, hybrids, and irons and so are shaft lengths. You really need to test on an open range or on TrackMan to dial in what will work best for you.

Why do want another club between the 4 and 2 iron? Dale is correct that you need to test them. The 7W will likely launch higher than the 5W so it will hold greens better

That said, it isn’t degrees but does the club fill a need for either distance or landing. You hear about blending sets. Most of us reach a point with one type of iron doesn’t hit our window and is inconsistent with variable roll outs due to too low of a descent angle.

It would be worthwhile to see a fitter to get the right tools in your longer irons an fairways. Guessing on a $300 dollar fairway that doesn’t solve a problem is an expensive mistake.