The only restaurant option currently with Nectar appears to be Pizza Express. The offer is £2.50 of Nectar points is worth £3.75. The Tesco Clubcard offer is £5 of Clubcard points is worth £10. In Sainsbury's you now need to spend £500 to get £2.50 worth of Nectar points where the same spend in Tesco gets you £5 in Clubcard points. So £500 spend gets you £3.75 at Pizza Express with Nectar and £10 with Clubcard. Even if you earned your Nectar points at the 2 points per pound rate £500 spend would only get you £7.50 rather than £10.
There's another catch with both the Nectar and Clubcard schemes. If you use your points to pay you're not eligible for any other special offers or promotions. Pizza Express often have generous offers on their website so it may be worth checking this before redeeming points. Also points can be only redeemed against food so you will need to pay by cash or credit card for the drinks.
In addition to Pizza Express Tesco also has a number of other restaurant partners all offering the £5 points = £10 value offer. This appears to be an ongoing offer with no listed expiry date.
Membership of the Gourmet Society gets you special offers and discounts at participating restaurants. It is possible to pay for membership with Nectar, but not Tesco. However, Gourmet Society is also an East Coast Rewards partner so if you still have points in that scheme it's an option to redeem.
Nectar charge 2000 points for an annual membership, that's £2000 spend in Sainsbury's or £1000 in other partners including Hertz and Virgin Trains East Coast.
East Coast Rewards charges just 290 points for annual membership, that's £290 in standard class travel or £194 in first class travel. Yet another example of why East Coast Rewards was the UK's best loyalty scheme. Find out more about Save East Coast Rewards.