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I want to increase water pressure and the amount of hot water available?
Is either option better than the other?
It depends.
Current heat source(s)
# of baths / showers / sinks
Usages pattern
We currently have a 20 year old boiler running on domestic oil.
We have one bath, one shower, 4 sinks, might install another shower.
2 adults, 1 six yr old.
What kind of flow and pressure do you get from your mains? If the mains is poor then you can go for an accumulator if combi or u/v cylinder or pumped vented cylinder set up. If its good, run a new supply pipe in from the mains perhaps. For combi boilers and u/v cylinders, performance is ultimately governed by the supply you have.
Quite a few options, with much variation on price and complexity. What are the current tap flow rates you're looking to improve on?
Assuming you have a good flow rate and the space for the set-up, I would 100% recommend an unvented system over a combi boiler. When we moved into our place 6 years ago it was the first thing we did. If the hot water is on then we can have an almost limitless amount of showers as the rate at which we use the water in the tank is lower than the speed at which the cold water can be heated. It isn't unusual for us to have both showers running at the same time (with *VASTLY* superior water pressure than we ever had with our previous combi boiler) and even if taps are running elsewhere the pressure is still usable.
It was an expensive installation but 100% worth every penny.
One bath and one shower? You could feasibly get away with a combi, that's pretty much the type of household they're designed for, and it would probably work out less than an unvented system even though oil combis seem to cost more than the equivalent system boiler - unvented cylinders are EXPENSIVE.
If you just want more pressure and more hot water at any cost, unvented system all the way.
Third option - a “storage combi”. Not cheap but in theory combines the best of both a combi and an unvented cylinder.
A small (in our case 60l vs about 300 for a normal cyclinder) cyclinder linked to a combi so it rapidly recharges. If you do use all the water in the tank it carries on delivering at combi rate (so you never run out of hot water).