Infected molar at bottom left jaw got two opinions from dentists. One said it is not salvagable and needs to extract. One said it can by doing a root canal and crown but he could not predict how long the tooth can stay. Don't know which advice I should

From . From your description, the tooth in question has an extremely guarded prognosis. You have two professional opinions; the first that the tooth is unrestorable, the second that it can be fixed, but long term success is questionable. Extracting the tooth and replacing it by another procedure (dental implant or fixed bridge) will be more expensive than fixing this tooth. Did the dentists discuss the alternatives and the costs of those alternatives? Only you can make the decision to restore or extract once you have all the information on your options.
Prognosis Important. The dentist that told you it could be saved, should give you a prognosis. That would be a categorical word like "excellent, good, fair, guarded, or poor". That gives you additional information on how well the "fix" could be done, under best conditions. In cases like this, depending on the prognosis, implants may well be a better choice. Depends.
Infected molar. Your choice is retain the tooth with significant expense of root canal and crown versus remove the tooth and place an implant--also with significant cost. With infected teeth already deemed nonsalveageable become brittle and break, and the tooth may be lost anyway--along with the funds used to "fix" it. More and more patients are opting to remove questionable teeth for the implant.
Extract and implant. If the dentist can not guarantee you five years or more with a procedure to save the tooth then you are better off with an extraction and dental implant.
Choose implant. If salvaging a tooth with root canal therapy...Crown is questionable....Then you should ask about the possibility of placing an implant to replace the tooth. Implants are very successful and a better solution than a questionable treatment.
The choice. Is yours--your dentist can tell you what your options are, and the usual expectations regarding how long the different choices with last, but your need to make the decision.. If you are unsure, get another opinion from another dentist that one of your friends has confidence in.
It's not always easy. First, no one, over the internet or in person has a crystal ball. If we did, we could & would always give patients the best advice. Two excellent dentists can agree to disagree as to best course of action. We also can't guarantee outcomes even if we do know what is best. Sometimes we just have to take a chance on whom we trust more, how much risk we will assume & just take out best shot.
Depends. An opinion is only as good as the doctor giving it. For an opinion on a root canal i would make sure it is given by a well regarded microscopically trained endodontist! they are the most knowledgable and proficient at these procedures. Their prognosis may differ from others bases on their ability and experience!
3rd opinion. Whenever u get 2 different medical opinions. Get a third one!
See a Prosthodontist. See the reconstruction specialist -- the prosthodontist -- for an opinion. If badly broken down, better with extraction and implant for long term success.
If . If it is a guarded tooth which it sounds like it is , you have to be ok with the idea that you will be dealing with this tooth again later and often times that means that the area around this tooth might have become more compromised and create a bigger problem. Since you are still questioning i would go to see an endodontist(a specialist who only does root canals) and see what he/she thinks the long term prognosis is. If poor, i would advise taking out and consider an implant or bridge for tooth replacement.
Tooth Repair. It all comes to the longevity of the proposed treatment. Nothing takes the place of a natural tooth. However, nothing can be more disturbing than spending lots of money and in 1 to 2 years lose the tooth. It's like buying a stock that you know in a couple of years has zero value. In dentistry, the goal is having healthy and functioning teeth and or replacements with predictable longevity.