As far as popularity is concerned, Pragraj is far popular than Purushottam
It is reliable and punctual train of NCR.
Purushottam, on the other hand, enjoys an upper hand in terms of latest rakes
If comfort is criteria, Purushottam is better
If...
more... railfanning is criteria, then Prayagraj, as there is more stops for this.
More stops provides more things to click and share (only if u plan to stay awake all nite)