The Sadducees - accepted only the torah (thus rejecting the resurrection due to it not being mentioned), they were a part of the priestly caste, the high elite, wealthy, and educated. Would you categorize them as being ultra liberal (As Tim Keller does), or as ultra conservative?
While the parallels aren't exact, there are some points that are rather interesting. The Saducees believed in a strict interpretation of the Law as originally written, and rejected religious innovation. It wasn't so much that they didn't believe that there were prophets after Moses as that they didn't hold them to be prophets in quite the same, authoritative sense as Moses was, and therefore anything they said was of secondary value. Substitute "Moses" for "The Founding Fathers" and "The Law" for "The Constitution," (or alternatively "The Bible",) and a picture emerges that's rather familiar to Americans who follow politics.
The Pharisees, on the other hand, were quite innovative. They understood that times change, and believed that the Law had to adapt to the progress of society in order to remain relevant. This is the core of their notion of building a hedge around the Law: making sure that people didn't end up doing things that would violate the meaning of the Law even in cases that the Law had not originally covered. The modern phrase "living document" comes to mind...
As I said, the parallels aren't exact, but when you look at the two groups' respective treatment of the Law, the Saducees come off as highly reminiscent of modern conservatives, and the Pharisees as liberals.
The disparity between those labeling them one way or the other is not due to there being disagreement over what the Sadducees and the Pharisees believed, who they were, or what they were like. Rather, the disparity is caused by a wishy-washy definition of the terms liberal and conservative. The terms liberal and conservative have many different meanings and can change depending on the context. They are also highly-charged political terms which can evoke unintended reactions. The variation in definitions for these words can be demonstrated by the fact that some people will say that Hitler was an ultra-conservative and some people will say he was an ultra-liberal. So who's right? The simple answer is, they are both right, depending on what you mean by the terms.
So, what exactly do the terms mean? Well, given the context that we are discussing here, there are really two particular facets to the definitions of the terms which seem to apply, namely:
When using the "justice" test, you would say that the Pharisees were conservative and the Sadducees were liberal. The Pharisees were so concerned with following the letter of the Law that they invented and religiously followed all sorts of additional rules to keep them from ever getting even close to breaking any of the laws. They followed these rules with no concern for mercy. The Sadducees, on the other hand, were seemingly often more concerned with their power and position than they were about strictly adhering to the laws.
When using the "tradition" test, you would say that the Sadducees were conservative and the Pharisees were liberal. The Sadducees treated the Torah as the most important and primary source for their religious beliefs. They were hesitant to put too much stock in the teachings of any other "inspired" works which came after the books of Moses. The Pharisees, on the other hand, followed many more modern traditions, some even at the expense of the original laws (see Matt 15:1-9 for an example).
In summary, both terms can accurately be applied to both groups, it just depends on what you mean by the terms.
This is a little bit based on differing opinions. However, ultra conservatives are typically seen as being concerned about every detail of every part of the Scriptures. The Pharisees appear to have been much more like this.
Ultra liberals are often seen as those who readily dismiss many of the teachings of the Scriptures, preferring some passages and general principles over specifics.
In today's world, some ultra conservatives may assert that any work on Sunday is strictly forbidden, even though that law applied to Saturday under the Mosaic Covenant and not the New Covenant. Some ultra liberals may assert that merely believing in God's existence is the only thing that matters and that we should just love everyone and not suggest that anything is immoral, despite clear teachings of Scripture that contradict this.
So, since the Sadducees disregarded everything but the Torah, that seems to fit more with those that show preferential treatment to certain passages of Scripture. This lines up more with being liberal than being conservative.
According to the New Testament itself the disagreement between the Saducees and Pharisees was on the resurrection. But Josephus adds that the Sadducees believed in free-will more than the Pharisees who held to a kind of fatalism although not as rigid as that of the Essenes.
Josephus Antiquities of the Jews, Book 13, Chapter 5, paragraph 9:
So conservative versus liberal here would depend on whether you think fatalism is liberal or free-will is liberal.