If you spend £15 a month, this is what you get:

	PAYG Minutes	PAYG Texts	PAYM Minutes	PAYM Texts
O2	100	100	200	400	With O2 you can double your bundle and save twice as much.
Orange	100	600	100	Unlimited	With Orange you get unlimited texts.
Vodafone	300	Unlimited	600	Unlimited	Vodafone offer double the minutes.
Three	20p a min	1000	250	Unlimited	You can double your minutes with Three.
Virgin	20p a minute	Unlimited	250	Unlimited	Only 100 minutes a month
T-Mobile	25p a minute	10p	300	Unlimited	Only 80 minutes a month with T-Mobile

Which Advice states –

PAYG mobile cons

PAYG mobile customers normally have to pay more per minute or text than pay monthly contract mobile customers.
You’ll probably have to pay full price for your PAYG mobile handset upfront.
PAYG users have fewer mobile phones to choose from than pay monthly mobile contract customers.
Some mobile operators may require a minimum PAYG mobile top-up to qualify for certain incentives (such as free weekend calls).
You have to top up every time your PAYG mobile credit runs out.

Pay monthly mobile contract pros

Pay monthly mobile contract customers get a wide choice of free or subsidized mobile phone handsets and a number of mobile call minutes and texts as part of the deal (depending on the mobile tariff you choose).
Some of the newest mobile handsets are available only on a pay monthly mobile contract.
A pay monthly mobile contract’s non-inclusive mobile call charges (for example to call 0870 numbers or when using mobiles abroad) can also be cheaper than with PAYG mobile tariffs.
You’ll never be left without a mobile phone service because your credit has run out.

Martin Lewis of MoneySavingExpert says –

Are you a Pay-as-you-go user?

Be very wary of picking a deal, just because the handset is cheap. Mobile phone companies often deliberately price sexy phone models lower, but on poor tariffs. So while you think "great, the coolphone101 is only £10 here" once you start using it, any savings made are soon eaten up by higher call charges; for everyone other than granny-in-the-glove box users, focus on getting the right tariff.