Markie,
Im not getting into a slanging match. I have a different opinion than you have , due to my experience in mobile phones. i think your opinion is totally biaed as you work for three. When i worked for vodafone i still thought t-mobile was best (a totally unbiased opinion).
I do know about cutomer churn, dropped calls, pricing etc etc
Most of it IS available for the public to see , especially price.
I find it a bit silly you think as you work at 3 , you know whats right and everybody else doesnt.
You say his opening statement was true. WRONG!, your opinion is that its true. The FACT is for value for money and download limits t-mobile are the best package overall.
Whether companies use different budgets, or they have had an agressive advertising strategy, doesnt make a single difference to somebody who wants occasional internet.
Therfore indeed any of the networks would suffice, but if its value for money then t-mobile is the best which is a fact.
For best network quality overall i would say three is up there at the top. Coverage is a postcode lottery, you know that. So what is absolutely the k9's undercarriage for one person is a nightmare of dropped calls, gprs instead of 3g etc etc.
So Spongebob, i would check which has the very best coverage in your area and choose that network (apart from o2).
Matt
Or your opinion of that facts

Though I agree with comment on use what works well for you.
As a consumer, for the areas I am in, o2 is far superior to anything else. Nothing works well at home except o2. We have had o2 since digital came out, and prices ok if you move tariff at end of contract - 2 o2 phones have cost me about £10 each this year on contract (100m/100t pm). Voda, my works phone, has decent coverage, but doesn't work at all well near my house. Prices, hard to say, as I gave up my old personal voda years ago, and the works one is on a corporate tariff, usually between £5 - £15 per month, including internet. T-Mobile, woeful, virtually non existent coverage between here and work (25 miles), glad i borrowed it before committing to 18 month contract (only way to get sensible price), hopeless. Frequently, even with reasonable signal, unable to use. Orange, better than t-mobile coverage wise around my way, similar priced package, but the better o2/voda coverage for similar money is a no brainer for me. t-mobile, similar money, longer contract, virtually zero coverage around here came in last for me.
No experience of 3, so cannot comment.