My view is that it really depends on the details of the share plan. If you have the option on whether to acquire the shares in twelve months time then it could be a no-brainer. For example, you save for 12 months out of net salary and you can have the option, but not the obligation, to buy the shares at a set price determined at the start of the savings period. If the share price is above your option price you exercise and, if not, you don't. Once you exercise you may be able to sell the shares straight away and realise your profit. If you don't exercise you get your money back. However if the plan requires you to use the funds to buy shares regardless of the price then it is a different proposition.
You will be liable to tax on any profit you make so you may need to sell some of the shares to pay the tax in any event. After realisation you can use the funds to invest in AVCs. So you may be able to do both.