When using the definitions posted by the OP Yield is a more important stat in a capper you want to follow.
Comparing yields will be more accurate and will also be easier to copy.
The definition of ROI used by the OP can lead to very strange records. If you follow a winning capper his most recent bets are going to influence his stats greatly. This increases the sample size you need to review. If you are betting a large bankroll yourself it might be impossible to copy his results because the liquidity to attain the same ROI isn't there. In addition you won't be able to withdraw from your roll to copy his results.
But having said that I don't like yield that much either. Especially if you are designing your own systems, there are many factors to keep in mind. Not just yield. For example $/hour and ability to scale up. I think for example for guys with small-medium bankrolls (small=me) trying to cap by actually watching games will return terrible $/hour.
-When determining to follow someone else go with Yield but adjust for reliability (sample size)
-When evaluating your own results go with (absolute $ - cost of capital) vs time invested