ISM uses five components, each weighted evenly at 20% — new orders, production, employment, supplier deliveries and inventories.
IHS uses a weighted average that gives greater importance to new orders (30%), output (25%) and employment (20%), and lower weighting to suppliers’ delivery times (15%) and stocks of purchases (10%).
-- Dion Rabouin, Axios
I don't know that comparing ISM to IHS ("Markit") is all that useful. Yes, Markit appears to lead ISM, recently (perhaps now, and certainly during May and June of 2018), but the S&P 500 has shown itself to be little-affected in the near term, by the transition in trend, in either ISM or Markit.
I track manufacturing ISM against the S&P 500 for a "big macro picture" data point, but as you can see from this monthly chart, the two do not move in lock-step. That being said, I like to be aggressive in the S&P 500 when the manufacturing ISM is rising, and not-so-aggressive when it is falling. If Markit is truly leading the ISM data, we should see a corresponding rise in the ISM, for December (which will be released in January). Does that seem likely, though? It does not to me, but let's let the data do the talking.