One of the common points one sees in a political debate is the equivalency between both sides. There's always one person who dismisses both major parties as hopelessly flawed - whether they call for a third party or for fundamental changes to our political system depends on the person in question, but that point is less important.
Of course, each side's partisans will quite happily blast such people for promoting what they see as a false equivalency. To them, it's quite easy to see that the two are not the same - that their side is better and the other side is worse.
I normally side with those that label it a false equivalency. Simply proving that both sides are flawed is not sufficient to prove that they are both equally flawed. The former is quite simple (and blindingly obvious, unfortunately). The latter involves digging through a wide variety of issues, scandals, and arguments to determine how many flaws there are and how much weight each should have. That's quite a bit more complicated, and very difficult to do without letting one's biases run away with one's judgment.
It's also very common to come up with some handy shortcut that simplifies the process. I don't necessarily disagree with the idea that once you've found an unacceptable flaw, or a benefit that you can't do without, then there may not be much point in wasting time trying to nail down more details. The problem is that not considering the rest of the data isn't a good thing; if there's more deal-breakers hiding in other issues, you might have to adjust your criteria for a deal-breaker or start figuring out who else you can turn to. It also makes it more difficult to defend your judgments if you haven't looked at some of the issues, of course (although given the wide range of topics, it's also going to happen at some point).
Better yet, it's possible for two people to run through the same set of data and justifiably come to two different conclusions. Just because I don't place the same weight on something like gun rights as a conservative person would doesn't make either of us right or wrong. That value judgment, like many others about the role of the government or the relative value of our rights, is difficult to adjudicate with reason. I can explain why I value something, explain why it matters, explain what else it affects... but in the end, it's still possible for someone to say that they know all that and point out that they still don't care as much about it as they do about other aspects of the issue. (For gun rights, the example would be me stating that more effort to reduce gun deaths may be worth increased restrictions on the right to gun ownership, while someone else might value the gun rights more.)
Even that is a point that can be argued, and it can be worthwhile to ask people why they feel that way, but such debates tend to stop being about data and start being about philosophy fairly quickly. In the end, the best way to answer the question about which side is better usually involves being specific about which criteria we're comparing.