ITI is a technical skill training which is based on more practical approach than theory and it last for 1 to 2 year. Polytechnic on the other hand is a 3 year course after class 10th which helps in theory as well as practical approach but theory is considered more. If you have time and money, I would suggest that you go for diploma otherwise go for ITI. Polytechnic diploma is preferred more and good Mark's can get you into good colleges as well. Also the pay for diploma holders is much better than ITI ones.