To be honest, your best bet is to find an IFA to provide you with some guidance on various pension options.
You could try finding one at: www.unbiased.co.uk or on one of the money websites (I think thisismoney has some links).
With your pension and any investment (rather than savings) you will have to pay for the management of the fund, with potentially no better performance than picking shares or funds out of a hat, which is why I guess you are looking at SIPPs.
The benefit of the pension/SIPP over an ISA is the 'grossing' of your contributions (adding back on the tax you paid on the income), which is something like an extra 28%, making your 300 = 384 per month.
From financialadvice.co.uk
SIPP tax savings
Contributions to Personal Pensions generate direct tax savings. Contributions are made net of tax relief, which means that you will only actually contribute £80 net for every £100 of contributions paid. Higher rate taxpayers likewise make contributions net of basic rate tax and can then claim additional relief via their Inspector of Taxes/Self Assessment return. A 40% taxpayer therefore only contributes £60 for every £100 of contributions falling within the higher rate band. These figures assume basic rate tax of 20% and higher rate tax at 40% (2008/09).
The FSA has said that in some cases people may be better off with a simple Stakeholder pension.
http://www.thisismoney.co.uk/pensions-tips-and-guides
HTH
Sparky