For rail travel it's worth comparing Nectar against East Coast Rewards the scheme that was in use on the East Coast mainline before Virgin Trains East Coast started in March. East Coast Rewards was regarded as the most generous loyalty scheme in the UK for rail travellers.
For UK domestic rail Nectar only allows you to redeem on Virgin Trains East Coast (Virgin Trains, the west coast operator, is excluded). Currently until the end of May there is a special offer where £2.50 of Nectar points is worth £3.75 in rail travel.
Tesco Clubcard has partnered with redspottedhanky.com and £5 Clubcard points is worth £10 to spend on any UK rail tickets (there's a £1 booking fee).
Even taking the booking fee into account the Tesco deal is clearly superior. You need to spend £500 in Sainsbury's at the new rate to get £2.50 of Nectar points (worth £3.75 for a limited time only) whereas £500 in Tesco will get you £5 of Clubcard points worth £10 for ticket purchases. The Tesco deal is not limited to any operator.
For European trips Nectar is a partner with Eurostar, this is the at the usual rate of 500 points = £2.50, there's no bonus.
Nectar partners with easyJet. There also used to be a deal with Sainsbury's where your points were worth more but that no longer exists. The rate is the usual 500 points = £2.50 there's no longer any special promotional deal.
Tesco partners with British Airways/Avios and Virgin Atlantic. The scheme allows you to transfer Clubcard points into airline miles at a reasonable rate.
250 Clubcard points converts to 600 British Airways Avios or 625 Virgin Atlantic miles this is a good deal if you collect airline miles for either of these airlines.
If you don't collect miles you can also use your points for money off a Monarch flight. £10 in Clubcard points is worth £15 off a flight.
Nectar allows you to book hotels as well as other travel through Expedia for the usual 500 points = £2.50 rate.
Tesco has a partnership with a number of hotels where £10 Clubcard points are worth £30 at the hotel.
Tesco is more generous with the travel redemptions too but both have relatively limited coverage in this area.
There's no real benefit using points at their face value (e.g. Eurostar, easyJet, Expedia and Virgin Trains East Coast after May) if they're worth the same as they are in the supermarket. It doesn't matter where you claim the saving you still end up the same financially, it's much better to redeem on something that offers a bonus.