I don't base on 5mm rounds or GW-type plastic bases. I think it makes everyone look like they are standing on a kerb. Instead I use 1" fender washers (I actually have about a thousand black oxidized steel washers I use), and occasionally 1.5mm acrylic bases (for motorcycles and horses). These keep the mini closest to the table top and blend away far better to my eye than the 5mm rounds.
28mm minis are poorly proportioned. By that I mean they are ridiculously wide. The biggest argument I hear for 1/43rd cars is that it looks like the minis can get in side widthwise. That is true. However I never put my minis in my cars - but they very often stand near them. So for me - height is the important factor.
28mm is a pretty inaccurate descriptor of miniature scale. Ideally the "X"mm refers to the eye height of a standing miniature - eye height is used to avoid confusion from hair, helmets or other headwear. Grabbing a couple EM4 Future Wars minis, it seems that they run to just about 28mm at eye level. Grabbing a few TAG, Artizan, Foundry and Copplestone minis (in upright standing poses) shows that most measure between 28-29mm at the eye. Taking that 28.5mm as an average and comparing it to reality - the average American male is 69.5" or 1765mm. Normal drawing proportions of a male body is 8 heads with the eyes located in the middle of the skull at 7.5 heads height. However the minis tend much more toward 6 heads in height... the Suit from EM4 (middle below) is 5.8 heads tall with the eyes at 5.3 heads height. If we assume he is of average height that puts the eyes at 63.5" or 1612mm. 1612/28=57.6 giving a miniature scale (based on height of the sculpt) of about 1/57.