Three's 24-month SIM only deals: are longer contracts worth it?

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We look at the advantages and disadvantages of taking one of Three's new 24-month SIM only contracts and how they compare on price. SIM Sherpa is your trusty guide. Last updated: 29th November 2019.
Benefits
Pricing: you save £1-3 a month vs 12-month deals
Most of Three’s 24-month SIMs are currently priced £2 a month cheaper than the equivalent deal on a 12-month contract, with the same minutes, texts and data.
Over a year, that adds up to £24. Over two years, it’s £48. And the discount is nearly uniform across all plans. Here’s the current pricing for each 12-month and 24-month plan:
On the 4GB plan, you'd save £3 a month against the 12-month contract price:
But we can actually beat both those with our exclusive hidden 12-month offer here:

Unlimited minutes + Unlimited texts + 4GB 8GB data = £13 £8 a month (12‑month contract)
See full deal
On the 12GB plan, there's a £3 monthly saving on the 24-month contract vs the 12-month one:
But again, we can beat both with a hidden offer too on a 12-month deal:

Unlimited minutes + Unlimited texts + 12GB data = £16 £10 a month (12‑month contract)
See hidden deal
On the 30GB plan, it's only a £1 difference between contract lengths:
On the unlimited data plan, it's a 50p monthly saving when you average out the cost of both plans over 24 months:
In the next section we look for any differences between Three's 12-month and 24-month SIMs.
Useful link: See all Three's SIM only plans
What's the same?
Three's 24-month SIMs have the same extra benefits
Other than price, there’s no other advantage to taking one of Three’s 24-month SIM only deals vs a 12-month. There are no added perks to reward your loyalty. And you’re not missing out on any either.
Each comes with Three’s generous Go Roam inclusive roaming scheme, which lets you use the majority of your regular UK minutes, texts and data in 71 countries abroad.
24-month plans with 12GB data or more also get you Three’s Go Binge unlimited streaming data too, which lets you use selected apps like Netflix without eating into your data (full details here).
And there’s other handy stuff too. Like WiFi calling, 4G calling and Three’s Rewards app called Wuntu.
Useful link: See all Three's benefits
Disadvantages
You’re committing to twice the usual max period for SIM only

✔ Do your research before entering a long contractCheck Three coverage
at Three.co.uk
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The reason Three are incentivising you with a £2 discount should be pretty obvious. You’re committing to a pay monthly contract lasting twice as long as the typical SIM only deal (24 months vs 12 months).
Getting new customers costs mobile networks huge sums of money on marketing and sales. So it’s way better for them if you join on a long deal, even if you’re paying slightly them less.
For you, it’s a trade off. You’re getting a cheaper price now. But you’re losing the ability to compare deals again in 12 months’ time to see if you could be better off switching providers.
It’s also worth thinking about what happens if you’re unhappy once you’ve joined Three. If you buy online, you have 14 days to change your mind. But beyond that, you’d have to pay to end your contract early.
So check Three have decent 4G/3G signal where you’ll use your phone (use their coverage map here) to make sure that’s not going to cause you problems. And read Three's terms to make sure you're happy up front.
Useful link: Three's pay monthly terms and conditions