Ricciardo and Vergne both seem really good (I'm in favour of Ricciardo, and not just because he's Australian), but neither of them have done anything to prove they're better than Buemi or Alguersuari.
Between Buemi and Alguersuari, I think Alguersuari has more potential for future growth and more natural talent to play with. I have a feeling that there might be a mid-season change in 2012.
I would say that but Buemi consistently is dominating Alguersuari in qualifying,
but one of them has to go and soon. They are basically equal overall and there needs to be another driver in the mix so you atleast have a benchmark, which is why i think one will go(most likely Buemi as Alguersuari is much younger hes even younger then Ricciardo).
Most of the commentators seem to think Ricciardo hasn't done much to impress in the HRT (hard to do, I know) and that Vergne is the hotter prospect at the moment.
Ricciardo has outqualified liuzzi 5 races out of 9, which compared to vettel in 07 when he was team mate with liuzzi couldn't even do.
Alguersuari seems to be an epic slow learner(was useless in 2009) and has been lucky that redbull have been as patient as they where with him. But if you put the two current str drivers in a redbull I can see buemi on top due to the fact qualifying means soo much more when at the front as the cars are more closly matched and that's the big strength buemi has over alguersuari.
Also on ricciardo, He's in a Hrt so the only feedback you will get on a driver would be qualifying as he hasn't exactly got a car to race anyone with.
Vergnes results in lower formula are identical to ricciardo anyway.
Either way Ricciardo and vergnes results in lower formula are much better then the two drivers they have in STR, and IMO have more possibility of emulating vettel.