First of all formic acid makes your mobile phase compatibile to MS. It could improve resolution in case of proteins or peptides because it acts as ion pair agent however not very strong. The disadvantage of formic acid comes from its higher UV cut-off comparing to phosphoric acid.

Concluding, I wouldn't say that formic acid is better then phosphoric one. It depends on what you want to do.

First of all formic acid makes your mobile phase compatibile to MS. It could improve resolution in case of proteins or peptides because it acts as ion pair agent however not very strong. The disadvantage of formic acid comes from its higher UV cut-off comparing to phosphoric acid.

Concluding, I wouldn't say that formic acid is better then phosphoric one. It depends on what you want to do.

Exactly, I also cant say that formic acid is better than phosphoric acid in HPLC. Both acids have their advantages as well as disadvantages. So, it is upto you that what you want in analysis.

For prep HPLC, I like formic acid if possible because it is volatile and can be easily removed. Phosphates require an additional desalting step.

As others have commented if you want to have a methodology that can also be compatible with MS then formic acid is a better option. In general I would suggest that you start with formic acid over phosphoric acid if possible as it just presents less limitations in transferability of methodology between systems. However in terms of the chromatography it is very dependent (as mentioned above) on what you are trying to achieve and the samples you are working with.

formic acid and phosphoric acid have two different contexts.

density of phosphoric acid is 1.88 where formic acid is 1.23.

pka value for formic acid is 3.75 where as for phosphoric acid is 2.12,7.2 and 12.3. so that you can use any of the pH for phosphoric acid, when adjusting to higher side.

But for formic acid its not possible.

Generally as above said formic acid is the best choice for MS/MS.

formic acid and phosphoric acid will behave differently in aqueous solutions and on stationary phase and with your compound. In case if your compound behaves same in both the acids, then its better to choose formic acid because of its volatility, if you don't have any peak shape issues

My experience says some compounds will give very good peak shape in phosphoric acid rather than formic acid because of its buffer strength.

If you have to run HPLC and LC-MSMS Simultaneously then formic acid is the best choice.

I agree to all. Both the formic acid and phosphoric acid acid have its own pros and cons while separating your compound in HPLC or LC-MS. It depends on your target compound which you want to separate. And moreover the choice of your column and mobile phase composition can also screen which acid you can use. My experience is based on separating the pharmaceuticals, is that the elution is sharp by using formic acid.

It is impossible to assume or speculate which acid (formic or phosphoric, or any other acid for that matter) is better for HPLC, simply because the selection of proper mobile phase composition depends on many different variables, and may vary considerably with respect to any particular application. The most important factors to consider while developing your mobile phase for HPLC application include: the compatibility of either acid under consideration with the other components of your mobile phase, compatibility with the specific analyte(s) you are trying to resolve, compatibility with the matrix, compatibility with solid phase, and the detection method. Development of HPLC procedures is a never ending process, and for all practical purposes it is an open book of chemistry. The best way to find out which acid is better for your particular application is the old and trusted trial and error approach.

Is it possible to get same profile [HPLC & LC],with orthophosphoric acid and formic acid respectively for polyphenols by altering strength of formic acid to suit that of orthophosphoric acid. 

Using formic acid in an aquous mobile phase, you might end up at pH 1 unless you buffer with a base like sodium hydroxide. Using phosporic acid you might end up below pH1.

What components do you want to separate, your question is not complete without more information.

Especially the separation mechanism you are using (stright phase, reversed phase, ion pair) is very important as well as the pKa values of the components you want to separate.