I second Max, there is a third option missing: I am interested in
both ladder and planetary.
A properly run league be it a ladder or a planetary assault gives a good platform to enjoy a mechwarrior game.
Some positive and negatives in both league types:
I spent the first 4 years of Mechwarrior4 playing exclusively in ladders from SL, VL, TWL and a short visit to P2K and MWA. SL and VL inherited a way of playing similar to the old MW2 ladders such as Grand Council on kali or MCWL on mpig, where players have to bid for each match. Although I enjoyed it in first 2 years of MW4, it was also a flaw. Having to bid for each match got irritating as hell after a while. Too much time wasted just to get a game going.
TWL where I brought my team to compete in MW4-BK offered the best ladder platform for MW at that time with a battle automation. There was no bidding, simply attack the target unit and the automation selected 3 Maps plus conditions of the battles automatically. MWL seemed to have inherited features from TWL since MWL's founder Neuromancer RIP (Brian Bisson RIP) used to be the shoutcaster of TWL-BK league before he founded MWL. I hope to see that kind of more efficient ladders like TWL and maybe MWL (if my picture of MWL was correct since I did not play MWL) in MWLL.
The last 6 years of MW4 , I moved on to planetary leagues (1 quarter in BZ, a few months VnR while playing NBT at same time.)
NBT style of league if run properly is very immersing and entertaining. A planetary league adds the strategy and planning factor into the enjoyment. NBT's extensive automation and dedicated servers were the strong points that made NBT stand out compared to other planetary leagues imo. And of course the cool roleplay

The one major flaw of NBT-HC: it has been running for way too long due to it being pretty much neglected by the main league admin some 2 years ago. NBT-HC has been running for almost 5 years by now, the league was never reset , which is very frustrating for a unit leader like myself as you get bloody stuck in that planetary league. I think every planetary league needs to have a fixed predetermined maximum running period. For a planetary as extensive as NBT, I would say a reset every 1 or maximum 2 years is needed.