If a players goes down but isn't on the official IR, ESPN will mark him as 'Out', which still makes him eligible for your IR slot so you can deal with the situation. I'm sure all of us have had some colourful words for Yahoo when a guy goes down but isn't on the IR (*coughGoncharcough*).
Hands down, no question, 100 percent, ESPN is the superior site. I've been in yahoo leagues for years, this season was my first being in an ESPN league. Any league I start will always be with ESPN from now on.
10 Team H2H ESPN Keeper - G, A, PIM, FOW, ATOI, SOG, HIT, BLK, DEF, STP, W, SO, SV, GAA, SV%
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Forwards: McDavid, Matthews, Ovechkin, Couture, Toews, Henrique, Perry, Perron, Maroon, Galchenyuk, Neal, R. Smith, Parise, Stastny, Konecny, Patrick
Defence: Ristolainen, Josi, Hamilton, Giordano, Fowler, Ekholm, Schultz
Goalies: Fleury, Lundqvist, Smith
IR: Crawford