I play demo defensively and it's my favorite class. Scottish Resistance all the way for me, no question. The extra stickies and the ability to selectively detonate them make up for the times when my demo is killed by someone while I'm prepping the field with stickies. I lay down all 14 stickies as soon as possible at multiple entrances or routes, on the enemy cart, or the capture point. Then I try to find a place I can keep an eye on these stickies without being shot (preferably near a friendly sentry that can cover me) and spam out 20 grenades at various rate of fire at entrances and other likely spots before I look for a dispenser or ammo resupply. The grenade spam supplements the stickies well and helps deny the area like only exploding ordinance can. Often I fire slowly when there are no enemies in sight, expending only one or two grenades out of the four before reloading. That allows me to be prepared to pump out 4 grenades at the enemy when I spot them and if they avoid my stickies. If they are standing on them I send a grenade that way and detonate on the launch of the second grenade (if I don't react and blow up the stickies immediately) The SR's point and detonate is great as you can aim directly at an enemy and only the stickies that are effective against that enemy will blow up, leaving some of your demo's sticky spread remaining.
If the enemy causes me to explode my stickies in a certain spot, I re-do those stickies until I have 14 unless I'm being shot at. The grenade launcher is much better for those situations when you are under fire. Direct hits from grenades are really strong and four can kill an unwary Heavy, and you can bounce them into cover for slightly less damage. Crit grenades are great in either situation. Once you've got the arc down you can hit consistently where you want them to go as far as they will go, if not the enemy standing there, who would have time to dodge if they are aware of the incoming.
I also like to do rolling explosions by starting on the right or left and moving over a line of SR stickies while detonating if there is a crowd on my stickies. It's as if the soon to be bits enemy are being pursued by explosions.
I enjoy launching stickies almost but not quite straight up so they rain down slightly in front of me. It's useful for landing them on a platform above you from a spot where the occupants can't shoot you without peeking over the edge to catch a grenade in the face while you detonate. If they don't peek over then you can launch a grenade up there after the stickies and detonate with the grenade. That's not really restricted to the SR though.
Then there is being able to remove level 3 sentries and any engineer henchman repairing them with only three or four stickies. The above tactic is a favorite for this. An engineer that can remove the stickies fast enough while I arc them from behind cover is a rare breed. Usually they rush me and sacrifice their sentry if they notice the stickies. It's worth it to me to have my demo die to the engi's shotgun if I can take out the sentry for my team; and the engi might eat a couple grenades so it's not guaranteed to kill me anyways. This ability in itself makes up for lacking air detonations when I go on offense in my opinion. Against other classes I tend to have to advance more slowly than the basic sticky launcher due to having to prep my 14 stickies to draw enemies back into, blowing those enemies up before I advance a bit more while setting up more stickies slightly ahead of my position. If someone interrupts I fall back and try to draw them over my existing stickies while launching grenades at them and ducking around a corner to reload or to bumrush with a melee weapon.
I tend to choose the Pain Train if I intend to capture or push a cart, the big demoaxe if I won't miss a small bit of maneuverability (like playing defensively in Hightower where I stand mostly in the same general area depending where the enemy cart is), a head taking sword if I plan to accomplish consecutive melee kills due to the map encouraging it, or Scrumpy if none of the above.
In other words, balance.