The "old" basing was originally a compromise to accomodate the load of figs I had based for other rulesets. Infantry elements are deeper than wide (20mm x 30mm), and cavalry is a different width (30mm). When I started basing figures again from scratch, and rebasing a load of Russians I'd bought on eBay, I thought I'd use the opportunity to experiment.
The "new" style puts both Infantry and Cavalry on 25mm square bases. This widens and reduces the depth of infantry formations compared to the "old" bases :
I also prefer the tighter spacing it gives to Cavalry :