This is a tricky question. PBMining has the lowest price per GHS so far and the website seems to run pretty smoothly. CEX has a higher GHS price but you can also resell your GHS anytime you want. I am running some numbers as we speak.
Let's create a hypo. Let's say that I buy 100 GHS on CEX and 100 GHS on PBMining right now.
I would pay 1.18 BTC on CEX at its current GHS/BTC price. I would get the same amount of GHS from PBMining at 0.7 BTC.
Let's assume that difficulty does not change, as this value does not really affect our analysis, which is focused on same amount of GHS mining roughly the same amount of BTC every day.
After 60 days of mining, CEX would have mined for me 0.506016 BTC (considering that CEX charges around 16% fee). PBMining would have mined for me 0.6024 BTC.
If you see these numbers you tend to think that PBMining is better. But if I stop mining for some reason on day 60, I can resell my GHS on CEX. Let's assume that the GHS/BTC value is 0.010 on CEX, I would then get a total of 1.506016 BTC (sold GHS and mined BTC), while on PBMining I would still be stuck with my 0.6024 BTC.
Let's say that I don't cash in and I keep on mining.
After 120 days, my profit with CEX would be 1.012032 BTC, with PBMining 1.2048. PBMining is still better but if I sell my GHS on CEX at, let's say, 0.007 I would get a total of 1.712032.
Any thoughts?