What?! the default gets absolutely tons of love. "stock is best and all". It's just for those who play soldier , the LnL is similar to how you would play with rockets so your prediction skills transfer over more easily. The damage variance means you can kill something in 3 pills for sure whereas with stock its MAYBE. The LnL also make it so I can kill a building in 2 hits from further away.
I personally also like the iron bomber just because I have more control and it doesn't bounce in strange directions, fits my style better.
MVM is different. Now the main role of the demoman there is to lay a trap so you can take out uber medics before they pop their charge. demoman is NOT the only one capable of doing so (and personally I play sniper instead of demo when needed because clicking on heads is easier for me by a long stretch), but traditionally he has been. grenade launcher in mvm is >> stickies when it comes to tanks and the playing style is more soldier-esque. It's pretty amazing. BUT if your team expects you to deal with uber meds, they'll be short a role thats more or less reliable. I think 2 demos (grenade and sticky) can be pretty efficient and fun in Expert for example (4 tanks) than the phlog pyro who TYPICALLY fails to impress unless against a tank (expert people like to throw the newbies on pyro because tank doesn't fight back). OH and since you are using a grenade launcher you can also use a shield for extra mobility and cheap resists, and change it up per wave . YEAP. I think the expert meta is really woefully blah. It works, and simple enough for newbies (2 heavies, 1 pyro and engineer. neither of which are complicated), but c'mon guys, seriously?
forgot to mention that grenade launcher while gives you the benefit of more control (shoot at bot coming at ya without arm time needed) it is not as, well, versatile as the scottish resistance in some points where you could detonate one trap out of 2-3.
In MVM you either get stupid people who can't common sense (thus simple meta needed), high tour people who are militant about the team composition , upgrade order and every little breath that you take. Or there are some cool high tour count people who are confident enough to see that what you are doing is not actually detracting and works just fine if not better. I wish on you all the matches where you get to be grenade demo and show people the beautiful fun robo destruction it is capable.