stable isnt gonna matter. theyre gonna be drilled and tapped for m4 screws on the bottom and drilled and tapped for m2 on the top to mount a flexdrive.
The one on he left is the better choice only because the spcing between the fins is much better and the fin tickness is better than the one on the right. you also showed the fins in a horozontal plane this could be just to show us your HS or not so I will say mounted in a horozontal plane would be best if you were using forced air cooling, if you are not going to use fans and plan on static air cooling you shoould turn the fins so they are vertical so that convection cooling can work for you.
what do you guys think? im waiting on my setscrews and hex wrenches before i list them but the machinist called me to pick them up today. its tapped for 2mm screws for a flexmod on top and tapped for m4 screws on the bottom for mounting and also has 2 m2 size setscrews.