actually "28mm" is a size and a proportion convention that has changed over time and is also subject to variation.
Applied to contraptions that claim to conform to scales like 1/56 or 1/48 (which is not always correct), it means that 1/56 fits the size of the 28mm miniatures, but usually has problems adapting to the oversized features, like feet, hands, heads and weapons (and equipment consequently), whereas 1/48 has no problems fitting the features (and the actual miniatures consequently) but tends to look too large compared to the true size.
the rest is preference and taste. The question is rather irrelevant if You do not have to fit the figures into the vehicles and becomes paramount with open vehicles like jeeps and halftracks.
Since it is an issue of relativity it also depends on the range of miniatures You use.
Example:
Perry miniatures fit perfectly with 1/56, since these are closer to real-life proportions
Renegade miniatures would fit better with 1/48
simple, isn't it?
