Hi Gareth,
Hope you are well.
All the phones you have mentioned are great handsets, so I can understand why you're struggling to choose between them. To help you with your decision I'll list the specs and the features of each handset and hopefully this will help you.
The iPhone 5 is the latest handset from Apple. It has a 1.2Ghz Dual-core processor, an 8MP camera and a 4.0In touchscreen. The iPhone 5 features Siri and Facetime like the 4S and is faster than previous iPhones as well as been taller and slimmer. The iPhone 5 also features the largest range of Apps available on a smartphone. The iPhone 5 will be a good choice if you've been happy with your iPhone 4S, would like a wide range of Apps as well as decent specs.
The HTC One is due to be released later this month. It has 1.7Ghz Quad-core processor, a 4MP Ultrapixel camera and a 4.7in touchscreen. The HTC One features Blinkfeed in which you can pick the social networks, news and feeds you want to stay updated on and they'll all stream live to your home screen. Also, HTC Boomsound combined with Beats Audio Technology gives an amazing sound quality. If you would like some of the best specs available on a Smartphone as well as having great technology for listening to music then the HTC One would be a good choice.
The newly released Sony Xperia Z has a 5.0in scratch and shatterproof touchscreen, a 13MP camera and a 1.5Ghz Quad-core professor. The camera also features Exmor RS so you'll still get a great quality picture even if it's taken in low lighting. This impressive handset is also fully dust and water resistant so if you want a premium Smartphone that's more durable than others as well as having a large screen and brilliant camera then this will suit you well.
And finally the Samsung Galaxy S3 which has been very popular since it's launch in 2012. It has a 4.8in touchscreen, an 8MP camera and a 1.4Ghz Quad-core processor. It features S-Voice which, like Siri is a voice activated personal assistant and Smart Stay, which uses the front facing HD camera to observe your eye movements and keep the screen lit up.The S3 is very user friendly and packs in some great specs. If you want a handset with a fast processor and some innovative features then the S3 will suit you.
I hope this has been helpful for you.
More information on the handsets can be found on the Phones 4u website. This will help get you started: http://www.phones4u.co.uk/sony/xperia-z/
Thanks
Emma