An ISA isn’t an investment in itself - think of it as an envelope.
A pension is also an envelope.
Inside each envelope is the things you’re actually investing in. Let’s say that the shares of Google, and you’re hoping they go up in value.
You can put Google shares in your ISA envelope or your pension envelope. (I actually have this: I have an ISA with the same investments as one of my personal pensions)
So - how do you choose which?
Both are envelopes designed by the government to help people to save.
Now, they want you to save long term. So to encourage you to use a pension envelope, they need to give you an incentive for locking your money away for money years. So - they give you the tax relief.
With an ISA, you can get your money back really quickly - instantly, from some.
There are lots of pros and cons.
As PP said, if your investments do well in the ISA envelope, that’s all yours to keep!
But with a pension, once you start withdrawing it, it’s just like a salary so you pay tax on it.
Going back to the poor returns...
Some companies offer a guaranteed return - a cash ISA. So they’ll say they will give you (say) 1.5% interest if you don’t touch it for a year. Better than the mattress gives. And it’s 0 risk as it’s guaranteed. But you can also have a Stocks & Shares ISA. Just as with a pension (and actually the very same shares!) you decide how risky you want to be. Generally the idea is, more risk - potentially more return! But, there’s no guarantee.
I have an ISA and a pension, because I want some money that I can access quickly in an emergency (the ISA).
In your case, you can access your pension at 55, so you might be happy with locking it away as it’s not for long. At 55 you could withdraw some - and pay tax just as if it was salary. BUT - this triggers rules about how much you’re allowed to pay into any pension in future, so don’t see it as a short term thing without understanding more!