You'd need to post more details about the specific scheme (what sort of discount, what tax treatment, what salary limits, what "get out" clauses if you choose to leave the scheme prematurely etc.) and your particular circumstances for people to comment meaningfully.
Presumably the pension would have better tax advantages? The main drawback being that you cannot access the money until retirement. Could you avail of both - e.g. benefit from the discount bonus promised by the share scheme and then put the resulting funds into a pension?