Buy original if it is affordable to you.
If you are on budget, do some homework and get 3rd party.
Some range of lens by 3rd party quality are actually on par or better than the original.
Supporting 3rd party occasionally so that they can survive in the competition.
This will prevent monopoly of the original manufacturer
i'm just saying that 3rd party MAY end up costing more in the long run.... and I feel that someone should highlight this so that the smart consumer can take cost of ownership and future compatability in to account as well and not just focus on IQ, purchase price and throw caution to everything else....
I will buy a third party if the price diff is significant enough and if I don't intend to use it v often... eg I'm thinking of getting a third party macro lense with a broken AF (cheap)because I don't take macro shots often enough to justify spending so much on an L macro lens. In this case even if the 3rd party depreciates to 0 the depreciation is still less than what the L will depreciate over the same period....
I am by no means rich but I will still buy L lenses because of the IQ, build quality and compatability with future bodies .... important since I intend to keep my glasses for 10 - 20 years.....
fyi... the newer L lenses really have godly IQ....
70-200 f4 with IS... all in a 760g package
16-35 mkII.... a zoom lens whose IQ comparable to a leica prime lens
it depends on what u buy.. really.. for eg, when i bought my tamron 28-75 it was 600, and the equivalent canon 24-70 cost 2000+ back then.. even if it doesn't work in future (which is still a long way to go coz i don't intend to change my body anytime soon), the cost savings i had is enough to make me go for 3rd party lenses. this is considering if i buy a original canon L in future, it'll probably be cheaper, and a few generations more advanced.
For the same amount of money, i would go for the greater focal range and aperture range unless the price difference is minimal.. it gives me more flexability and chances to learn... i don't usually pixel peep anyway so as long is the IQ is reasonably good I'm fine..
Canon EOS 30D w/ grip 580EX, Tamron 28-75mm f/2.8, EF-S 10-22mm