I currently have a Nexus 5 (will be upgrading to either the HTC One M9 or iPhone 6S later this year) on the SIM only £15 per month 'All-in-one' Add-on with Three (I own the phone outright).

Is it worth signing up to a 12 month contract for £15 per month? I get a weak 4G signal with Three and EE have much better coverage and faster speeds in my area but are a lot more expensive (£28/month for their max 5GB data package), especially considering that they don't even offer unlimited data.

As far as I can see, the advantages of the contract deal over the Add-on are a 4GB tethering allowance, unlimited texts and free calls to 0800 numbers, however you lose 100 minutes (200 vs 300 with the Add-on) although this shouldn't be a problem as I use an average of around 150-175 minutes per month.

What is the better deal?

0800 is going to be free calling from all mobile sim cards, even 3's PAYG later this year.

In effect the 0800 coming to the PAYG will make it the full 100 minutes extra on Three's All In One 15 vs Three's contract £15 per month, so soon you won't be eating into the extra 100 minutes on the PAYG vs contact by having to call an alternative number to the 0800 number.

So really with Three and their £15 per month sim packages it will be a simple question of do you need the extra 4GB in internet tethering that the contract sim offers, or do you need the extra 100 minutes that the PAYG sim offers?

EE just don't offer unlimited data, so that is a no go for many. Three's 3G is excellent where I live.

I'm not too worried about the extra minutes or tethering so I'll likely stay with the Add-on for the time being in case anything changes as I'm not tied in for 12 months+ at least. I don't fancy switching to Virgin Media either due to their aggressive fair-usage policy, despite having their broadband.

Would you consider Vodafone at all?

I would need the Red or Red L package (£22/£27 per month respectively) with my data usage. They have decent 3G coverage however no 4G yet in my area, nor in the next 3 months according to their website. I'm not sure that they offer much of an advantage over Three considering the extra cost to be honest.

giffgaff (runs on O2 network) they offer 4G but again look on O2 website to see coverage

https://giffgaff.com/goodybags-4g

1,000 UK minutes
Unlimited UK texts
5 GB Internet (Max)
Free to giffgaff numbers

https://giffgaff.com/goodybags-3g

3G

£20 from 1st April 2015
2,000 UK minutes
Unlimited UK texts
Unlimited Internet
Free to giffgaff numbers

But Three dos sound what you need to be honest if you like your 4G

I used to be on Giffgaff until I switched to Three a while ago due to their constant service outages and price rises, coupled with O2's poor 3G which completely ruined it for me. I think it's a good idea in theory however it leaves much to be desired in practise.

I know you clearly state that you didn't wont to touch Virgin Media, Virgin Media customers for bb and tv etc you get like a discount but again tethering clearly states that is not a must and might get in trouble depending on the mood of the person who catches you, Also keep in mind that number to virgin media landlines are free from virgin mobiles so keep that in mind too aswell but since is all unlimited is free anyway,

Monthly cost Minutes Texts Data
£20 Unlimited* Unlimited Unlimited
30 day rolling (so if they did block you for tethering atlast is not a 12 month contract)

Small bit (The legal stuff)
"You cannot share your data connection with other devices - known as "Tethering" - or use your phone for peer to peer file sharing. Acceptable use policy applies. Calls to 08 (incl. 0870 and 070) numbers not included in monthly allowance (other than those included in the unlimited 08 tariff ) and cost up to 41p and £2.50 a minute respectively in the UK (see our tariffs for details). Pay Monthly service terms apply.

Pay Monthly SIM Only: 30-day minimum contract applies to Virgin Media cable & National customers, 12 month minimum contract for Non-Virgin Media customers."

So basically Three are doing the same as above for £25 all you can eat data/texts/mins on pay monthly or Pay and Go £28 with 4G