Berglund is more skilled and on a more competative team...so his +/- will be way better. With keeper implications I'm hesitant to do this b/c Berglund has routinely been compared to M. Sundin. A. McDonald won't be around forever and Berglund may well be the eventual #1 C on that team.
If I was just looking for points the rest of this season I would do this. STL rolls their top 2 lines too evenly for anyone to establish elite fantasy relevance. Berglund is only playing about 16 minutes / game, right now sits 9th on the team in PP TOI [although 1 through 9 are admittedly close] and has an unsustainable shooting percentage. I guess I'm saying he's not the go to guy in STL.
Versteeg on the other hand played close to 20 minutes/game last season and was the leading F in PP TOI and I think he'll get back to those levels as he re-aclimatizes from injury.
I'm probably in the minority, but if you are most interested in points this year I would take Versteeg.
9 Team [keep 10 + 3 prospects]
Scoring F=1[g/a]; D=2[g], 1[a]; G=2[w], -1[L], 1[OT], 2[SO] Top 12 F, 6 D, 2 G count
Crosby, Ovechkin, Backstrom, Iginla, Pavelski, Ribeiro, Marleau, Huberdeau, Moulson, Lupul, Wheeler, Pacioretty, Fleischmann, Weiss, Ennis, JVR, Burrows
Byfuglien, Letang, JJ, Carlson, Kulikov, EJ, Pitkanen, Faulk, Giordano, Orlov