Depends on what you want to achieve... I found that such a layout is VERY rolly, so a ball with a higher RG is a good base (saving energy for the back end), unless you want it as a smooth roller for long and heavy oil. Some good RG. diff. is also not wrong - too little, and the ball won't turn much at all.
From both balls, I'd give the Zr40 a try, and adjust the surface if needed. The Tantrum is more modern, but with its low RG I'd only favor it if you were really a low revs/high speed player. The Zr40 - by its numbers - seems to offer more versatility and room for tuning here.
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DizzyFugu - Reporting from Germany
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