V30... more agressive mids and broken tempered glass highs, chunky low mids... more definition than greenbacks.
Greeenbacks, very little bottom which is slightly muddy, sweet vocal like mid hump, shimmering highs which get sweeter after breakup... sweeter and creamier than V30's.
Both are great for Rock, Classic Rock and some Metal... V30 defined 80's crunch rhythm and Greenbacks defined the 80's singing lead tones.
For example Iommi uses Heritage Greenbacks.
Greenbacks were my favorite speaker until the Eminance Lynch Super V ( based on the Vintage Fanes ). Hiwatts still come with Fanes... another excellent speaker.