F-16 Block 50 is indeed a totally new aircraft compare to the old versions. However, i still think that J-10 has something that PAF prefers and that is why it is considering to go for it.
Its VERY simply webby, dont get side tracked by nationalism, the real reason why Pakistan is going for J-10 is geo-politics. J-10 is a good fighter, but its CHEAP, there is no fear of SANCTIONS, there would be favourable terms of payment, it would also curry some favour in China. Its not more about capabilties as the things mentioned above. Its certainly a good fighter, but in all honesty its like a Mirage 2000 now, which can be developed a lot better in the future blocks, its just the starting line for J-10 now, but what we tend to forget is by the time new blocks come, the world will also advance to the next stage. J-10 is the PERFECT LO to the F-16, there is no doubt about it. It makes a LOT OF SENSE for Pakistan to buy J-10.
J-10 can face any aircraft. Sukhois, F-18s, F-16s, without any problem. It is no wishful thinking. Hell if it couldn't, why would Chinese not just acquire some more Su-30s. What makes you think that J-10 cannot compete with these fighters?
Its VERY VERY simple again webby. J-10 is developed by China, they want to promote their own a/c, no matter how it is now. It will give a tremendous boost to their indegenous aviation industry which will in turn spew out even better fighters in the future. That is why. India will go for LCA no matter how bad it is, it is again nurturing your own industry.
Chinese already have a lot of Su-30's to meet any threat. Now what they need is to promote their own fighter, any country in the world would do the SAME thing, there are no exceptions.
The J-10 can compete, as any fighter can with any other plane, but it cant match the above mentioned planes 1 for 1 by a good margin.
BTW, the Su-27 was old, becuz the Chinese are trying now to upgrade their Su-27's to some SMT like standard that is for the MiG 29's.