Now I'm going to guess that this is a 1-yr league seeing as you're even considering this. If that's the case, I say pull the trigger. Williams will get more points this year.
However, if this is a keeper league (IN ANY WAY SHAPE OR FORM), you NEVER trade Seguin for Williams, regardless of cats or pts struture. Seguin is simply to valuable to trade for Williams because he is going to be an elite player in the NHL.
10-team Auction League, $160 team salary, weekly H2H, start: 6F, 4D, 1G and 5 Bench
Scoring structure: (F : 1pt G / 1pt A / 0.15pts SOG / 0.15pts Hits / 0.15pts BlkS), (G : 5 pts W / 3 pts SO / 0.01 pts Saves)
Forwards : Ovechkin ($43), Kessel ($20), Benn ($7), Spezza ($13), Brown ($7), Marchand ($13), Landeskog ($3), Elias ($1), Plekanec ($1),
Defenses : Letang ($26), Visnovsky ($1), Bieksa ($1), Faulk ($2) Ehrhoff ($2)
Goalies : Holtby ($7), Nabokov ($2)