F-35 is multi-role, for the integrated battlespace, primarily air-to-ground sphere. PAK-FA is aiming to become multi-role but with lagging Russian PGM and air-to-surface sensors/missiles and set to replace the MiG-29/31 fleet foremost and then Su-27 fleet (keep in mind 120x Su-34) and for India MiG-29/Mirage 2000 complementing the MMRCA/Su-30MKI fleet, focus will be (or remain) on air-to-air. I say remain, because it promises (arguably) more capable BVR missiles (ultra long range) and WVR sensors and performance.
But there's only few cases where they could compete. India being partner, could eventually be interested in F-35, but it won't a competition, because for India it's almost always also about tech. transfer and production/maintenance by HAL, so a small off-the-shelf fleet of F-35 might be possible but not more than that until US/NATO has a replacement for F-35 anyway. I believe PAK-FA is likely to be offered to South Korea, as alternative to its own program, but there's no way South Korea would opt to go that route and F-35 still seems the preferred option, in fact South Korea has more faith in it than any other nation. Then there's Malaysia, but it's too early to tell. And Indonesia in many, many years, which was reportedly in negotiation with Korea to join their fighter program, so it's the only country I see that could end up picking the Su-xx (T-50 is just the OKB designation) over F-35... although I doubt it will be any time soon that they will be given the F-35 option with many, many wories over the country's stability and regional balance. But they've been offered the F-16 now, so who knows in 15-20 years time.
The market is really non-aligned or CIS nations, and most of those nations will never go for the F-35 or be allowed to buy it.
The most likely scenario to me still seems it taking at least 20 years even before we see ANY operators besides Russia/India. By that time, the world will have changed a lot.
Strategically, rather than export, it's more of a concern. In that respect, I'd be more worried about its capable BVR missiles than the platform itself. The way Russian radar tech is developing, especially in range and sphere, rather than resolution, and missile tech they've already got, it's not the F-22/F-35 that is under threat, it's the freakin' force multipliers (awacs, jammers, tankers) that NATO depends on which become under threat when facing stealth PAK-FA data-linking packages. By the time the F-35s are in range to deal with the threat, they will have been taken out, leaving the F-35 as "blind" as the T-50... meaning WVR combat, where the T-50 certainly has an edge.
But I'm not the worrying kind of person some are, and I don't see it ever happening. But no doubt, T-50 is Russia's insurrance while the strike and bomber fleet will do the dirty work. It won't be much different for any buyers. That's a big difference from F-35 customers, who need it to take part in the US/NATO battle sphere.
Just my 2 cents, nothing factual.