It really depends on what your reason is to save. If it IS for a pension, than invest in a pension. You will get tax relief on your contributions directly....not by having to claim back at the end of the tax year but directly every month into your pension pot. eg ::
You put in £80 a month and the Inland Revenue puts another £20 in. So you get £100 a month going but you only pay £80. Its a simple as that. If you are a higher-rate tax payer, they put even more in for you.
No other savings scheme gets this extra FREE money. There are restrictions on when you can access the pension that you have been saving up for and currently its 55. The main advantage of saving into a pension rather than an ISA, apart from the free money, is that you can't take it out after a few years and fritter it away on something trivial. So its disciplined and you get the extra contributions. When you get to 55, you are able to take take 25% of your total pension pot as a tax-free lump sum, to spend how you wish.
So, for example, you get to 55 and you have a pension pot of shall we say £100,000. You can take 25%, ie £25,000 and do what you like with it, as long as the remaining 75%, ie £75,000 is converted into a pension for life. You don't have to take 25% as a tax-free lump sum if you don't want to. You can take less or nothing at at all. You can also enact all this at a later date if you wish but not before age 55.
If you save for your retirement via an ISA or any other cash saving scheme you don't get the lovely tax-free contributions every month. But you can spend it all on whatever you wish. To use my example, you might have £100,000 in your ISA when you reach age 55, or any other age and you can buy a retirement pension with this money if you wish. So you will end up with a pension after all. But the only contributions going in each month have been your own money, and none will have come from the Inland Revenue.
So tax-free contributions and a tax-free lump sum with a pension plan, but you are restricted in what you can do from age 55.
No tax-free contributions with an ISA but complete freedom on what you can do with the money.
The choice is yours. The extra contributions from the tax man every month are very attractive and the discipline is not bad thing, as an income in retirement, ie a pension, was what you wanted all those years ago when you started saving.