While you're testing, the sampling rate (which you didn't mention) can make much more of a difference than 16 vs. 24 bits.
In any case really I wouldn't "cheap out" on SD cards. You've already wasted more time on this problem than any possible savings is worth.
Not only is the ostensible class of the card a factor, but there are also counterfeit cards around, which makes whatever the label says worthless. Buy a well-known brand of card of appropriate specification from a well-known, reliable dealer and get on with your recording! Normally the R-44 is rock-solid reliable.
--best regards