Yep, sims will be PAYG and shouldn't need any topping up and will come with numbers ready to use.
The signal bars are a good (if not the best) indication of signal strength.
The best test would be for someone to call you whilst you walk around the house/office and see if the signal drops in...
I'd take the results there with a pinch of salt.
There are a few things inside your house or office, for example, which may interfere with your signal which that map won't show you.
The best method is still the 'suck it and see' option.
Try out both sims for a day and see which work best in...
I'd pick up 2 sim cards (Virgin & Tesco/o2) from a supermarket for £1.00 each.
Put them in the phone and see which gets the best signal at home and work and then make your choice based on that.
I was with Virgin before EE was about, so I can't really comment on how it is now, but in the past...
I should add that I'm currently half-way through a 24 month contract with Vodafone.
I'm in Hove (Portland Road area - BN3 5) and am yet to have any signal problems.
Customer service has been great for me, however you'll likely find just as many horror stories with CS as you will praise.
For...
I've been with the lot over the last few years.
Best three by FAR are: Three, Virgin & T-Mobile (NOT EE).
Worst in my experience: Orange & o2 (inc. Tesco, Giffgaff - but these are purely due to network speeds).