prime or zoom telephoto for motorsport?

[rsvnigel]

Particularly the Canon 300mm f/4L IS USM or the 100-400mm f/4.5-5.6L IS USM, which would be paired with a 30D.

I'm leaning towards the 300mm prime as it's a couple of hundred grams lighter, and faster. With a 1.4x converter is also as fast as the zoom at the long end.

Both roughly the same price and I have a £105 cashback voucher for each.

Any thoughts from the panel, my other lenses are the 24-105 f/4L, 50mm f/1.8 and the 28mm f/1.8?

TIA.

[Andy M]

I'd go for the prime. Have used neither though, so my opinion is limited.

[poah]

I'd go for the 100-400. the 300 f4 is a great lens from using it but its not got much versatility

[monkeyhanger]

Get the 100-400. It will make an excellent partner for the 24-105.

Remember going from 300 to 400 using a convertor takes time and runs the risk of introducing extra dust into the camera.