Whatever model you choose, how do you define 'saving money'? If it's headline price then, yes, you'll probably appear to save money on the net... and if nothing goes wrong, then you will be better off. But if you've got any foresight, you should at least consider factoring in the following:
1/ The fact that you won't try on the trainers before buying, and won't get first-hand advice... this means that you are more likely to accept something that doesn't really satisfy you. Doesn't give you the pleasure. Maybe isn't an absolutely perfect fit - or as good as some trainers on the next shelf, that you'll never get chance to truly consider. (I'm not talking about gross problems here, because you could return those, I'm talking little niggles). So, on average, people will replace these items bought on the net sooner and, IMO, that typically should add between 15-40% to the true cost.
2/ Depending where you buy, then once in a while, you'll get fake running shoes off the net (last time I looked, even ebay's help section was offering advice on how to spot them!)
3/ You have the messing about with returns etc, which are never easy... and from most websites cost you postage to return. It's sometimes a bit messy getting back to a shop, but the chances of needing to return to a shop are much much lower.
The trouble is, people never really factor things like this... they buy on headline price. Some will end up happy customers... some might be blissfully unaware that they've bought sub-optimal products. Some will just not be as happy as if they'd gone to a proper shop. Is that really worth feeling like you've saved a tenner on headline price?
And I say a tenner, because the internet mega-deals are often mainly beautiful bits of marketing. People are conditioned to believe the marketing hype of low low prices on the net... without realising that they are often little/no different, except for clearance stock... and often forget that in-shop sales rails will also have great deals on clearance stock too - something that is often forgotten when people compare prices.).
Basically, I recommend you visit you local SPECIALIST sports / running shop. Get good service, a fair deal, a product you'll get more wear and pleasure from. It might not pay off for every purchase... but on average, in the long run, you'll be better off. (and no, I'm not a retailer!)
Edited: 01/10/2012 at 23:15