JP, there are a number of reasons why pricing can differ greatly.
It's always worth trying to look beyond the "marketing" speak to determine what you're getting.
Cheaper pricing could be an indication of cutting corners, higher pricing could be an indication of more money invested in the service provided, remember to ask probing questions, offline if necessary.
I'll make some general points not relating to any one particular provider.
A provider may boast a redundant BGP multi-homed network with many peerings but they may not (for instance) carry any spare infrastructure kit or maintenance contract (this was demonstrated recently by one provider - 48hr outage).
If a provider talks about having multiple quality upstream providers, do they actually route most of the traffic via those providers, or their cheapest tier-2 transit supplier?
http://www.traceroute.org/ is a useful tool for checking.
A provider may be selling at, or below cost, to gain market share (having done the maths on some WHT offers, it just doesn't add up).
I've heard of some providers offering initial signup pricing, and then implementing monthly increases, worth checking to make sure how long the pricing is fixed for. Also check the pricing for any additional service upgrades - you could be stung later.
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