Happy, since you mention the $6K Groceries cap on the Preferred card and question whether it's worth it... I'd suggest that, at least mathematically, it is

If you value MR points at a minimum of 2 cents each* AND you believe you can spend $6K/year at grocery stores... off that spend alone, you'd glean a value of:
- $360 on the Preferred card (6000*3*0.02)
- $240 on the regular card (6000*2*0.02)
...and that's without hitting the 20 or 30 purchase tiers.

If you DID hit those purchase tiers (and personally, this seems like a royal pain in the posterior, particularly for the 30-purchase-tier of the preferred), you'd earn respectively off just the $6K grocery purchases:
- $540 on the Preferred card
- $288 on the regular card

So in either case, it looks like it's well worth getting the preferred card if you plan on hitting the $6K grocery spend.

*If you value MR points at less than 2 cents a piece... then you may be better off with the Fidelity Amex card, which pays a straight 2% cash reward on spend with no mileage-redeeming hassles. But personally I think MR points are worth well over 2 cents a piece, at least when redeeming for miles used to procure international business-class travel; my last MR redemption weighed in at ~10 cents per point (Singapore Airlines premium travel .