No matter which B School you choose, PGP is always a better choice than ABM. But there might be a specific reason why you got an ABM offer rather than a PGP. But at IIM Lucknow, students of both programs are offered equal opportunities. These include final placements. summer placements and subsequent PPOs. 

The major drawback of ABM is that you are deprived of the PGP electives and can cover ABM electives only. These electives cover strategy marketing and operations. 

Talking about your offer from SP JAIN for IM, let me tell you that in this regard, IIM Shillong is a much better choice. Its campus in Dubai is a testament to its worth.