Of the two, for pure infantry skirmish, I’d go with 28mm.
For my own Vietnam gaming, I opted for 20mm, but wanted to include helicopters and/or vehicles from time to time.
I recently reconsidered the 20mm/28mm question with the new 28mm figs that we’ve been seeing on TWW, but don’t want to give up the slightly greater scope of games that 20mm permits on my table.
My main concern regarding 15mm was related to ease of handling of 15mm individually mounted figures in highly detailed jungle terrain. 20mm still gave me the range of scope that I wanted, and would be a touch easier to manage. That is also my primary consideration regarding your question. If using individually mounted figs, I’d go with the larger scale, particularly if you are just doing small skirmish.