If those are the two rounds your having the issues of choosing between then lets step back and look at some other reasons to choose one over the other.
Brass quality ,
338 Lapua , the most consistant and strongest brass available , 100 pieces with likely last the life time of the gun , and at $2 a wack I hope so , only two known makers and a few factory loaded round to choose from.
300 Dakota , pretty consistant brass , about $1.50 a case and only Dakota makes it , its kinda soft like that from Norma so its not gonna last much more that 8-10 firings I would think.
Balisticaly the 338 will win likely with both light and heavy bullet as the light bull;ets can be driven very fast , like the 200gr Ballistic tip driven to 3300+fps , talk about making a mess of a deer !! yet you can load it down to shoot in the 338 win range if your shooting deer under 400yds or you can loaded it up with the 300 gr SMK and make a good 1000yd elk rifle , can't say that about the 300 Dakota.
Me personaly I'd go with the Lapua
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