Bilstein B4 or B6 on stock springs/height?

Hi

I'd like to stiffen up the suspension a bit on a mk1 4-door without lowering it. The springs are in good condition so I'd rather keep them, the shocks from 1976 are sloppy. I'm considering Bilstein B4 or B6 with the original springs, would anyone recommend one or the other? Thanks
 

I've decided on the B4's after a bit of reading..Will report

What have you concluded about the B4s?

The car turned into a complete resto, so I haven't tried them yet! Still at the welding stage..

And how did they turn out, 4 years later?

opps