Honestly, both institutes have their own legacy and are excellent in the finance sector. Yet here I am writing some major points in favor of IIM Lucknow based on my cousin's experience.
Placements: The placements at IIM Lucknow for finance have been doing exceptionally well with the highest CTC standing at INR 53.34 LPA and the average CTC at INR 32.21 LPA. A good number of front-end roles, corporate finance roles, and foreign markets desks based out of India were offered.
Faculty: The top-notch faculty offers the best classroom learning backed up by Guest lectures and Industry workshops to promote maximum exposure.
Learning: IIM Lucknow offers a wide range of electives to choose from based on your interests. Some of them include Investor Psychology, Quantitative applications in finance, Project Finance, Corporate Banking & Risk Management.
SPJIMR is also a good choice for Finance but from what I have read, many major firms that visit IIM Lucknow don't visit SPJIMR so this might limit your opportunities to a small extent.