I need some new sunglasses for cycling - what features will actually be helpful - I've seen ones with vents, polarized lens, rubberised frame, replaceable lens . I just need the best value model that looks good.
Definitely something plastic, as that's going to be both safer and lighter than glass.
I'd look for something that was polarizing, as that will reduce glare, and something that offers UV protection.
You also should be aware that different colour lenses suit different light conditions, so you might need to make a decision based upon when you normally ride. I note that you live in London - I'm in the UK too and given our frequently poor light conditions, for the majority of the year I ride with clear (but high contrast) lenses.
Here's a colour chart I found a couple of years ago, to give you an idea.
As for replaceable lenses, they're a great idea but make sure you have an idea of the costs involved first. I got some glasses a couple of years ago and found that the cost of the spare lenses was disproportionate to the cost of the glasses themselves. (But still cheaper than 2 pairs of glasses.)
Another thing would be fit. You don't want them moving around as you cycle - I made the mistake once of buying some nice glasses mail order, when they arrived they were too loose on my head.
Lastly, if you come the the same conclusion as I did about using clear lenses, consider safety glasses such as these. They hit the same quality standards as bike glasses but are a fraction of the price. They are more fragile than cycling glasses but if I go through three pairs a year, its still cheaper.
Aside from that, how much do you want to spend? The pros seem to ride Oakley Radarlocks at the moment but, for what they are, the price is staggering.
I prefer yellow lenses to clear, for night riding.
Another important aspect of fit is if they are too tight a seal around your cheekbones then they will tend to fog. I was very annoyed when Serfas changed their Hunter lens shape without renaming. When I bought a second pair online the new lenses come further down my face and make enough of a seal that the lens fogs until I have some airflow. This is a safety issue - if I'm riding hard in cold weather and stop at lights I have to lift my glasses to see clearly.
Here in Perth, Western Australia, I use a rose lens for the part of the year where I'm riding in mixed light in the morning. I dislike changing lenses frequently so have used two pairs of glasses in the past - keeping my yellow ones for night or early-morning commute and a much darker pair for afternoon ride home.
A stupid owner of a bike shop said to me, "See these glasses - they cost $300 a pair and they weigh XX grams."
I said holding up my glasses, "See these, they are industrial safety glasses, they protect my eyes from ultraviolet, stones from cars and trucks, everyone stocks them, they are nearly impossible to break and they cost $7 a pair."
This was my way of inferring that can have excellent eye protection in clear and tinted and I don't look like a wanker who has more money than brains.
From my experience, you need to make sure that the top rims of the glasses above the lenses don't get in your line of sight while you have your head pointed downward otherwise you'll suffer neck ache from craning your neck upwards to see out of your glasses.
I found that was the case with non-cycling specific glasses but my Shimano's are just fine.
Worth keeping in mind as it's not obvious and nobody seems to talk about it.
To avoid the color choice predicament, I always look for cycling glasses with interchangeable lenses! Then, one pair can be clear, yellow, or gray (or rose!). I have had great luck with Smith Optics glasses.