For Demo, I’d recommend sticking with the stock loadout, because it’s overall the best for every situation. Demoknight is fun and all, but it’s far inferior to stock.
Your primary objective is area denial. Lock down important spots with your stickybombs and harass approaching enemies with grenades. Practice with directly hitting them with grenades before they bounce so you can kill them easily (eventually getting to the point where you can use the Loch n Load). If you’re holding one specific area, such as a flag or capture point, you might want to use the ScoRes to have better bombs. It’s also more useful for holding down two or more separate corridors, because you only have to blow one set.
Beyond that, you’re primary objective is to trash sentry nests in defended areas by loading stickybombs and blowing them simultaneously for enough burst damage to beat the Engie’s repair. You also need to keep close to Medics, as usual, because Demos are amazing when ubered or kritzed.
Sticky-jumping is a risky, lethal form of super-advanced mobility. Place one or two stickies, hop in your desired direction, detonate, and go flying. If you know the map and your distances well, you can get up to sniping nests easily and rain hell from your stickies and grenades from a protected area.
Finally, you need to know that you can charge up stickies to get more distance if you hold M1 down before launching. Helps to blow sniper and sentry nests from safer areas.
Soldier, on the other hand, is a more aggressive playstyle. You should usually be spearheading the offensive. I’d recommend sticking with the stock loadout, or using the Direct Hit and Escape Plan with the Shotgun. The Rocket Launcher is better if you’re dealing with dodgy enemies or if you don’t have the precision to use the Direct Hit effectively, but eventually the Direct Hit will become amazing.
You’ll be constricted pretty heavily to mid and close ranges, to a slightly lesser degree if you do go for the Righteous Bison, but it’s hard to use effectively at a long range, so try to get your enemies in enclosed areas. It’s important to manage your ammo so you’re never caught reloading both your shotgun and launcher, as reloading time is one of the Soldier’s main weaknesses. Switch to your shotgun to finish hurt enemies if you run out of rockets, or vice versa.
If you’re in smacking range, use your Shotgun and get meatshots or rocketpunch them with the Direct Hit if you don’t mind eating some splash damage. Soldiers are pretty lethal in close range, and can close the gap easily if they can rocket jump and propel themselves off wells correctly. It takes some practice, but it makes the Soldier very mobile, especially if you’re overhealed or don’t care about taking some damage.
You’ll also be on sentry-smashing duty if your Demos are absent, and it’s fairly easy to angle your rockets to splash sentries out of their LoS. The Direct Hit is especially effective at taking sentries down, since there’s very little possibility of a miss.