Thomas is more of a party line Justice than Scalia by far. Scalia has issued pro-liberty rulings in the past, it just has to be something you can demonstrate through the literal text of the Constitution.
I'd say this is both true and false depending on what you mean by 'party line'.
Thomas is, as far as I can tell, the more consistent with regards to his constitutional interpretation. He is a harder line originalist and textualist than Scalia by far, and so ends up to Scalia's right on occasion.
However, Scalia is more of a partisan ideological individual. He is far more likely to take a position inconsistent with his stated beliefs regarding the constitution and interpretation when it allows him to take a position in line with his political beliefs. Thomas is not perfect in this area but he is nowhere near as blatant about it as Scalia is.
I have far more problems with Scalia. Not only is his partisan ideology more dangerous than Thomas' (admittedly absurd) originalism alone, his conduct is rapidly making the Court itself look like a joke. The only justice I know who is similarly outspoken is Breyer (who I have seen speak) and he comes across as more of a grandfatherly professor. Scalia comes across as Bill O'Riley. And this is becoming more pronounced in the last few years.
A good example of this;
Scalia and Breyer debating their views of the constitution back in 2007. These are the two great bomb throwers and ideologues of the court, who filled such a role even then. Yet the debate is reasonable, both make strong cases and good points and it's a really good watch. Then you have Scalia's absurd behaviour this latest term, with open attacks on the president in his opinions and his interviews being hardline partisan in nature.
It's worth noting that Alito, who is being tipped to fill Scalia's role as the right wing anchor from this term onwards, is pretty much Scalia without the pretence of originalism.
This could be a good thing or a bad thing, but at least it gives us some good lines;During argument in Brown v. Entertainment Merchants Association (the "violent video games" case), Alito interrupted a Scalia question to say, "I think what Justice Scalia wants to know is what James Madison thought about video games. Did he enjoy them?"
On the health question, Scalia doesn't seem a likely candidate for Obama to replace. Everything I've seen suggests he is in good health for a man of his age (weight isn't really a good indicator by the by). I wouldn't be surprised if he even outlasts whoever comes after Obama, or at least gets a term into their turn. He certainly is going to give it a go, and I honestly don't see him voluntarily stepping down without a strongly conservative president in place to replace him.
On the other hand I wouldn't be surprised if Ginsburg steps down voluntarily after the midterms, especially if the Republicans look strong then. There is no way she will want to risk being replaced by a Republican and her health problems are serious and recurring. I think she could probably last out five or six years, but the risk of having to stretch to another eight or even twelve is too much.