Re: 7 mm lrm
Interesting topic!!! I'm tagging in....
Short answer: These two are ballistically very similar so other factors must be used to arrive at the most balanced decision.
Since you're probably going to be reloading, the next consideration is the ease for sourcing components. In this instance, the most critical component (as AZShooter states) is brass casings.
Unless you prefer beltless cartridges (such as the 7mm LRM), the 7mm/300 Win Mag chambering uses readily available 300 Win Mag brass which makes it the best choice.
If do you prefer beltless chamberings, then the 7mm LRM is the better choice.
On another note.... A while back I was also in the research stage for a "zippier" 7mm chambering (other than my current 7mm Rem Mag). I read a lot about the 7mm STW/RUM/Dakota, but those were all a bit too hot for my liking.
The 7mm Mashburn/300 Win Mag/LRM all met the criteria I was looking for.
Essentially, for me the sweet spot for my intended use (up to elk-sized game at max. 600-700yds) was a good 7mm bullet in the 160gr-180gr wgt minimally at 3,100-3,200fps. The 7mm-300 Win Mag for me is the best choice due to ease of sourcing components (I already have 500 300 Win Mag cases).