I am of the firm opinion that a Bank Guarantee (BG) is infinitely better than a post dated chq PDC.
Actually, a BG cannot be between the employer and the trainee, The BG will be issued by a bank, on behalf of the trainee, in favour of the the employer. It is easiest to invoke a BG - just write a letter to the Bank demanding payment within x no.of days, and the Bank is bound to pay, within x no.of days, without even considering the merits or legality of your invocation claim. Consult a lawyer to draft a water-tight BG.
On the other hand, there is no guarantee that a PDC, when presented for payment, will get passed, Then you are left with options like sec 138. But, if the PDC is a security chq, sec 138 does not apply. Proceedings in courts take a long time, and you may lose the case on technical and other grounds. Please see the posts in Forum>Criminal Law>Cheques and Forum>Business Law>Banking - you will understand the practical difficulties in winnigna sec 138 case.
The only thing that you will lose if you decide in favour of a BG is that you as an employer may lose out on a no.of talented but poor staff. Such BGs will be issued against 100% cash margin by banks, which mean that the trainees will have to furnish Rs.1 lac FD to the Bank, on the basis of which the bank will issue a Rs.1 lac BG. Trainees from poor families may not be able to furnish Rs.1 lac FD to the bank and so will not join your Co.