Cartridge advice

For ’12 inch tonearm this "SP" (Special) version is a killer.
XSV/3000SP compliance is 15cu at 10Hz, and this is the only Pickering top of the line cartridge with lowered compliance for modern tonearms, the rest of them are twice as much in compliance. For $400-500 NOS it is a steal! Stereohedron diamond tip (long life for over 1000 hrs). Try vintage MM - they are the best for the money and outperform almost everything twice the price. Higher models (XSV/5000 or low impedance XLZ/4500 and XZL/7500) for lighter tonearms are even better. 

Don’t fret too much on stylus type. I’ve a Decca with an elliptical and it blows away my AT750 Shibata. The generator is important too.

Your Decca does not have a cantilever at all, it’s a MI design and very special in it’s own way (the lack of cantilever is a goal of Decca), but this cartridge is not for every tonearm. Garrott brothers retipped Decca for years for their clients (with better stylus profile) and it was huge upgrade.

The problem with Decca MI and all MC cartridges is re-tip service, with simple profiles like elliptical the owner will have to re-tip every 600 hrs. With MicroRidge profile a cartridge can be used for 2000 hrs. With Stereohedron profile over 1000hrs for sure (or even longer). Profile is important anyway.

Pickering XSV-3000SP has the most advanced stylus shape called Stereohedron which is achieved by grinding four flat surface on the diamond at precise angles to each other and their intersection creates areas used to contact the groove. The advantage of the Stereohedron stylus is that because of it’s long and narrow contact surfaces it tracks high frequency modulation minimizing groove wear.

Expert Stylus & Co. in UK was Stanton/Pickering diamond supplier, they are still in business, their new profile called Paratrace. This is the best company for Pickering/Stanton service (if the owner can’t find an original spare styli). As far as i know Decca owners often re-tip their Decca cartridges with Paratrace profile in England.