Well you really need to ask yourself exactly what do you want to use it for.
If you want to really push your photographythen i'd say spend the extra now to save later.
The non IS
is from most reports at FM
's a slightly sharper lens.However the IS
on the 2.8 is worth a minimum of 3 stops and is a great piece of technology allowing shots like this.
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Photo © Darren Howie
in itelf is handy for windy conditions to allow you to maintain a low ISO without blurr etc.
At the end of the day IS
is a huge advantage which comes to the fore inpoor light but nevertheless is a handy aid even in reasnoble light.