[cynical on]
I know, lets turn it into an Eldar Cobra, with a Leman Russ battlecannon - or better still, a Volcano cannon - as its secondary armament

[/cynical]
Guys, isn't this kind of discussion at the heart of the debates that have plagued the Tau development over the years. First statement is that there are two variant models, so there must be two set of stats. Next statement is a question of whether these can be balanced or not and thus whether they can get the same cost. Then you get debates on the strengths and weaknesses of each (which quite frankly depends on too many 'battlefield factors' as Asuara suggests), and quotes from various 'fluffy' sources to back up one position or the other, and we go round again.
Perhaps we could approach this slightly differently by pricing for the "best" stats (which most seem to think is the Railgun variant) and allow a "free swap" for the other one. Permitting 'mixed' formations would also allow the stats for each variant to be made slightly more AT or AP specific while letting the player determine how he wants the formation to perform.
Final thought on costs, what do you guys think of the notion of having a cadre of 5x HHeads for 325, and allowing upgrades of one HH @75 points?