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What level is your party? 4? How large are they and are you planning on throwing any really tricky stuff at them? Do you think they're equipped to handle that or are they going to have to be creative? Is there room for creativity? How generous have you been with gold/items? Are the enemies really well geared or about average? What are the environmental factors like?
The experience rules of thumb work pretty well but these factors do affect things significantly; for example you'd be sending a level 4 party up against a CR ~7.x ish encounter -some of the possibilities in that range are really difficult for a party to deal with if they're inexperienced/don't have the right tools- which is extremely difficult and can cause a wipe depending on any of a number of factors. Compound that with possible traps, or gimmicky items/effects and things can be nasty.
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This depends on whether they have to fight both groups at once, their level, if they can bypass one or the other without fighting, and if they've had to blow a lot of resources on the previous encounters which are critical to success at this stage.
Mobs vs the single really powerful dude: this depends a lot on the party. It sounds like we're talking low levels so probably your melee and ranger types don't have a lot of iterative attacks. So vs mobs it's pretty much going to be up to your casters.
Mobs: This means that unless you have one or two people with a lot of cleave type abilities that your party's entire ability to fight mobs/swarms is your caster(s). Hard to say in general terms if they'll still have some good aoes/ccs left at this point. I don't know your players. If they don't then mobs and swarms are a great way to wipe a party, especially if any of them have some gimmicky abilities (shadows for example are CR 3 and extremely strong against a poorly prepared/magic poor party). Generally too, mobs overwhelm tanks since there are just so many more bodies coming than they can reasonably block, and few players take combat reflexes so expect combat to hurt your squishy types "in the back" more than usual. You can compensate for these issues a bit as the DM if you think it will be a problem by leaving appropriate loot in the previous encounters/elsewhere in the dungeon and fixing the terrain so the party can be sneaky. Generally these encounters stress your healers and support types because they have to constantly worry about different people, who might be spread out over a large area, going down simultaneously.
Single Targets: These are great for melee and ranged damage heavy parties. You have your tanky types gang up on the bbeg and restrict his movement and everyone slowly whittles them down. You can better accentuate your strengths as a group because the squishies are more or less free to do their own thing and generally the healer only has to worry about one or two guys going down. Conversely in a heavily caster party single target encounters can be very difficult at early levels. They have some cantrips which can whittle down the evil guy (assuming he doesn't have resistances), but if they have had to blow their load getting to the boss they're screwed.
So really it just depends a lot on your party members and the composition of the specific encounters.
