in all honesty if im doing any grinding i still tend to opt for high variance games with lower stakes, if allowed (some dont) i go for mega-spins break da bank again (simply to do it quicker), and play on low stakes, or 243 liners with 0.60 bets,
depends how much is at stake currently and how much wagering you have to do, but at least i know with high variance i might lose more but i have a chance of increasing it also, on 0.60 spins i did about 2k's worth of wagering on IR and finished 300 up, thanks to 2 good bonus rounds
i've done 14k spins on bloodsuckers to grind out a WR and it was the dullest hours of my life, but even that slot had the balance rising and falling (very slowly) over several hundred pounds on min bet (never again)