That depends on your circumstances. GiffGaff aren't great if you're a Pay As You Go (PAYG) customer who doesn't use many minutes or much data. I include myself in that description as I use very little of either as people generally call me and have wifi. PAYG rates if you don't buy a goodybag in the UK are very expensive compared to Three. https://www.giffgaff.com/pricing vs https://www.three.co.uk/Store/SIM/Pay_As_You_Go
To keep costs down on GG you have to buy a goodybag starting at £6 to get all calls/SMS + 500MB. That expires though after 30days and you lose your data and minutes. With Three the credit doesn't expire so long as you make a call or use some data every 6 months.

The goodybags also don't work in Switzerland, Monaco, Jersey, Guernsey and the Isle of Man. Those are part of the 'rest of the world' countries. So you need to have airtime credit which will be depleted by 5p a text, 15p a minute and 20p per MB. So expect to pay £20.00 for 100MB. Going outside the EU (the point of this thread) say to the USA but excluding Hawaii and Alaska. It is still 20p/MB but calls both making & receiving are £1/minute. For data in Hawaii and Alaska you'll be paying 25 times that at £5 per MB On Three it's still* 3p/minute**, 2p/SMS** & 1p/MB in both States as well as the rest of the US.
*or out of your allowance if you've bought a top up etc.
**outgoing only - incoming is free.

Disclosure: I'm a Three 321 PAYG customer.