Should I upgrade my iPhone 4 to a 5c or 5s?

I have had my iPhone 4 for a year and a half, I got it cheap on ebay an it works fine with nothing wrong. But I think the model is getting quite old now as it came out in 2010 and I want a phone which is lighter and has a bigger screen. I really like the iPhone's interface and I'm quite sceptical of buying a different brand as I might be disappointed with it. Is the 5c or 5s worth the money? And if so which one should I get? Thanks for your help.

Added (1). I know a few of you are very keen tech enthusiasts but I don't know why the 5s would be a tiny screen? I'm quite happy with the 3.7inch let alone 4inch! Also some of us don't just have £600 to spend on a phone (I only spent £320 on a new laptop) and contracts in the UK are more expensive than in the US so I'm on pay as you go.

-Yeah, get a 5S if you don't want the 6. Forget the 5C.

From the iPhone 4, I would say if you have the budget for it then go with the iPhone 6 or the iPhone 6 Plus. However if you don't have that much then go with the iPhone 5S. The 5S, 6, and 6 Plus will stay relevant for longer and will receive updates for longer without f**king up your phone. Plus the three of them that I just listed off have fingerprint scanners, and better cameras. They also are remarkably faster than the iPhone 5C.

Hoped I Helped. Peace!

Honestly. I personally loved my 4s. The ONLY feature 5s offers is the touchy fingerprint scan

The 5c is old at this point, definitely rule that option out. It really depends on what size you want your screen to be, as both the 5s and the 6 (and 6+) are lightnigh fast and will be for the foreseeable future. The 6 has a 4.7 inch IPS display (IPS as opposed to super amoled which Samsung uses (just a bright screen pretty much) and the 5s has that awkward shaped 4 inch screen. They both have a pixel density of 326 PPI (pixels per inch) which means the average eye can't discern any individual pixel. They both have 1gb of ram (dissapointing for us tech enthusiasts the note 3 has 3 GB)but apple is well optimized and it shouldn't be a problem. The cameras are pretty much identical but the 6 is most likely going to have a small edge. Lastly the 6 is newer and will have a little speed and support (as in how long apple will support iOS updates for it) but you can save money with the 5s and if you like tiny screens the 5s is for you. I would always go with the newest one if I were you.

With 5C free on 2 year contract, and 5S only $79 on con (walmart), no reason not to upgrade.

I upgraded from an iphone 4. I bought it from Apple with a 2 year AT&T Contract. The phone is amazing. It is so fast compared to the 4. All of my APS boot up with lightning speed. The screen is clear, crisp and the colors great. The battery life is about the same as my four. I live in an area with very poor cell phone reception. It is my feeling that the 5s handles that better than did the four. Areas that I could not make calls from on my 4 are doable with the 5s. I have read many negative reviews from various sources, but my own experience has been nothing less than positive. In my mind it is a great phone and well worth what I paid for it.