I don't think it is fair to either batsman to strip down to statsguru nitpicking.. I have seen enough of both to understand they were a cut above the rest in my time of watching cricket.. If you consider both formats of the game, I think Sachin comes in ahead with a clear albeit not exactly a comfortable margin..
But talking tests alone (applying CW's unstated law of Tests only if not specified otherwise), I still think Lara is the slightly better batsman of the two. Both of them are equally good against mediocre bowlers and great bowlers.. Both of them are equally good on good pitches for batting and bad ones. The difference would perhaps be in their style of play alone, with Lara prone to being more flamboyant while Sachin is prone to being more cautious. Both methods have their own advantages and risks and it is really silly to try argue one is better than the other, esp. because Lara had to do the one man show a lot more than Sachin (again, nothing to do with either man's talent but just the circumstances).. Lara was more likely to wrest the momentum away and help out the batsman at the other end that way, Sachin was more likely to look secure and solid and help out the batsman at the other end that way..
The reason I rate Lara higher than Sachin is that I believe in test matches especially the ability to carry on for the really big knocks matters when you are in good form.. That is the only area where I feel Lara pulls ever so slightly ahead of Sachin as a test batsman as every other factor between them, to me, evens out at the end. That, and the fact that Lara was the lone ranger for his side and I feel such batsmen carry a bigger load than others. But that is not Sachin's fault and it is not fair to pull Sachin down because his peer batsmen were good..
Again, I would like to refrain to get into stats mudslinging now to bring up scorecards and show how many times Sachin was the only one to have done well. Well, he is an ATG and that is what you would expect him to dominate scorecards no matter how good/bad his peers were in the said matches.. I am simply thinking of instances when I looked at the team sheet and felt "Oh God, get Sachin/Lara out, this team is done." It happened plenty with Lara but not so many times with Sachin. Those are my reasons.