Dangerous question to ask me
The 15y is a good beverage... I really like the newer 18yo right now. I don't like to give advise (spending your money) without bases (i.e. what your looking for). But with a list of bottles and prices, I can give a recommendation (have had most of the standard bottlings)
I am not sure what series you are talking about... but I have had a few 12yo springers in various cases:
Springbank 12yo 1995 (56.1%, Original Bottling, Cream, /592, cask 96/274, d:6.95, b:4.07) Springbank 12yo 1995 (55.3%, Original Bottling, Amantillado, d:6.95, b:4.09, cask 96/255, /577) Springbank 12yo 1996 (56.3%, Original Bottling, Fino, butt 96/262)
They were all good. But not better than a standard 15yo at about the same price. They are interesting enough to try if you have a good price. Cask strength helps, and they will give more of a cask influence.
There is a new 1996 14yo series out right now (was not sure if you meant these).
Cheers,