need help with rear shock.

i have a saracen raw worth £ 250 - £300 and the rear suspension is in need of replacing, i have found that rock shox make a unit called aero 2.0 which is 167mm in length from eye to eye and ranges upto 200 mm in length, the measurment for my frame is 170mm. now i am stuck and am in need of help with which size unit will be better 167mm or the next one up which is 180 mm?
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Contack MOJO or TFtuned for a replacement shock.
Parktools :?:SheldonBrown