Hi Kopsis,
As far as I'm aware, display legibility in bright sunlight is a problem regardless whether you use a cycling computer or a smartphone. Certainly some will be better than others but both groups include good and bad. The only really good solution to that (IMHO) would be an e-ink display. Do any cycling computers offer one?
Battery life is an excellent point that I overlooked. I train for distance and ride centuries so this is no trivial matter for me. On my second century this year (2014 Apple Cider Century in Three Oaks, MI), I was approached by a cyclist whose cycle computer battery had just expired. Regretfully, I couldn't help him. If I remember correctly, he was doing either a 50 mile or 75 mile route. His route had joined the 100 mile route that I was close to finishing.
Regardless, whether a dedicated cycling computer or a smartphone is used, the battery life can be important if you are a long-distance cyclist like me. My daily training routes are each 35 miles long and take just under 2 hours.
In my case, I lock my display on rather than off (with the "No Screen Off extended" Android app) because I like to monitor my progress and compare my pace and heart rate to a previous workout while I'm riding. I also like to watch my speed because I often push myself to go faster on the downhills. In addition, I turn my display up to max brightness for a daylight ride but I allow its light sensor to dim it automatically if I ride into shadow. My smartphone's battery is bigger than most. In some respects it needs to be because my phone's display is huge. It can handle a workout with no problem even with the screen turned on the entire time and four radios running continuously (4G, GPS, Wifi, Bluetooth).
But I replaced the stock 3100 mAh battery last year with an after-market one that has twice the capacity (6200 mAh). It requires a special battery cover to accommodate its larger size. I can run an entire century if I'm careful---but it's tight. So I added an external battery/charger with a 15000 mAh capacity. I carry it in a bag behind my seat and it connects via a USB cable that I attach temporarily to my frame when its needed. I use it to trickle charge the battery in the phone during the century ride and it gives me a comfortable margin. But I have to manage it wisely because most smartphones cannot charge the battery as fast it is being drained when four radios are on AND the display is locked on and set to it max brightness, too. So you have to begin trickle charging at the beginning of the century and basically slow the drain.
I don't know if such flexibility is available for most dedicated cycling computers but they are for most smartphones. You rarely need to be limited by the stock battery, regardless of the phone. At the very least, you can connect an external battery via USB like I do on my centuries.
Back to the OP, the primary point that I hoped to make is that a smartphone is a viable alternative to a dedicated cycling computer. For me, it is a superior alternative. And, since most of us have one anyway, it can save money to use it instead of buying another gadget. The OP asked for recommendations on what computer to buy. Even if the OP doesn't have a smartphone now, other readers probably do and may want to consider using it.
Finally, their is one circumstance where I wouldn't use my smartphone. Raiden reminded me of it---heat. If you ride where it gets really hot (I'm originally from Florida) or worse---hot and humid---then I agree with Raiden that an expensive smartphone is not advised. Besides, most smartphones have "smart" charging circuits that monitor the temperature of the battery and reduce their recharge rate when they get hot in order to protect them from overheating. My external battery/charger option wouldn't work under those conditions. And the batteries become less efficient when hot so you probably won't get as much life out of it. If I had to deal with high heat, I'd opt for a cheap cycling computer. But I don't have that problem. I've lived in Michigan for over 25 years and, while it can get hot in the summer here, too, most of my riding is fairly cool. And if it does get hot, I ride early in the morning (often starting before sunrise). Most of our centuries are in the fall when our biggest concern is rain and wind.
You have to pick a solution that will work best for you and your budget.
Kind regards, RoadLight