The main difference between Digging Spades & Border Spades is size, and this differs in two ways;
1. Head Size
Digging Spades generally have a bigger digging head than Border Spades. As a very rough rule of thumb, the digging head of Border Spades is around 30% less than that of Digging Spades.
Digging Spades are also typically longer than Border Spades. An approximation would be that a Border Spade would be between 5cm & 10cm shorter than a Digging Spade.
Both of these combined therefore means that Border Spades are lighter than Digging Spades, with maybe around a 10% difference in weight between the two.
So which are better - Digging Spades or Border Spades?
Well, the honest answer is "it depends". The use of a spade is very much a personal preference, however, in general Digging Spades are more often used where a large patch of open ground requires digging, whereas Border Spades are used where space is more tight - typically between established plants in a garden border or vegetable patch.
However, if you're not able to lift heavier weights or you have a tendancy to over-do it by piling too much onto your spades, then smaller Border Spades may be the better option for you. Female gardeners often find that Border Spades are more suited to their needs due to the lighter weight of the spade itself & the smaller soil loads lifted.