I would go the 7mm-08 it has much better BC for projectiles of similar weight over the .308, since you don't have a 300win mag yet I would go 7mm rem mag so you can use the same 7mm projectiles.
I reckon the 7mm rem mag is better than 300 win mag due to better BC of projectiles
Not quite right, true the BC of the 7mm projectile is better than the 308 but the 308 can usually be driven faster. I ran the below 2 loads through Applied Ballistics using velocity data from ADI and Berger projectiles as they use the more accurate G7 BC.
7mm, 168Gr Berger Match VLD Hunting G7 BC of .316 @ 2486 FPS 1:9.5 Twist (Load data based on ADI manual for 168Gr SMK 37.1Gr BM8208)
30Cal, 168Gr Berger Match VLD Hunting G7 BC of .242 @ 2707 FPS 1:12 Twist (Load data based on ADI manual for 168Gr SMK 43.3Gr BM8208)
200m Drop / Energy
7mm-08 2.37moa - Energy 1791 ft-lb T.O.F .28sec
.308 1.98moa - Energy 983 ft-lb T.O.F .26sec
400m Drop / Energy
7mm-08 9.37moa - Energy 1370 ft-lb T.O.F .60sec
.308 8.28moa - Energy 1405 ft-lb T.O.F .57sec
600m Drop / Energy
7mm-08 18.28moa - Energy 1028 ft-lb T.O.F .97sec
.308 16.83moa - Energy 963 ft-lb T.O.F .94sec
At all distances the 308 had slightly less drop and the edge on energy out to 400m but the 7mm-08 retains more energy out past 400m due to it's better B.C. Important to keep in mind that a 168Gr projectile may not be the best performer in either calibre, I used it because the 7mm-08 load data tops out at 175gr projectiles and 208Gr for the .308 and 168Gr is reasonably easy to find. The math shows that performance is very similar between the 2 rounds to the point of insignificance till you want to shoot past 400m. The ballistic differences / drops / energy remain fairly consistent across a few other loads I checked for same weight projectiles. I possibly should have scaled the projectiles for a fairer comparison, as in a 155Gr 7mm Vrs 168Gr 308 but it's late and I need to get up early. If I was going to choose between the 2 I'd probably sway towards the 7mm-08 for a couple reasons. First up is that performance is comparable to the .308 and secondly it does it with 6.2Gr less powder. I've had a 300WM and sold it, I have a .308 and rarely shoot it cause when it comes to the 30cal stuff I love my MkV Weatherby Accumark S/S in 300Wby Mag. I believe it has a slight edge on the 300WM, it's different and I prefer to be the Sheppard and not the sheep.