I had a quick look at the older admob plugin, for iOS it uses version 7.1.0 and for Android it uses whatever latest version is.
The C3 mobile-advert plugin uses 7.24.1 for iOS and 11.2.0 for Android. Note that AdMob doesn't have a unified versioning system between the iOS and Android version, so it's not that on iOS a really old version is being used.
Admob seem to be requesting 7.0.0 or higher for both which is a little confusing, but both plugins should fulfil this requirement. Either way I'm going to look into changing the mobile-advert plugin to use the latest version ( we're only a couple of months behind but it makes sense to keep up to date ).