Akash,
I would say that you would always opt for the standard integration unless you've identified - via a thorough data discovery session with key stakeholders - that you have requirements that it cannot meet. I can also share that I was a marketing automation consultant for 4 years and implemented many, many Eloqua/SFDC clients and we ALWAYS used the standard integration. In some cases, we used it very creatively, but we used it.
I also had the questionable pleasure for one client of integrating Eloqua and a third-party provider's SFDC instance via CastIron - in my opinion, it's not worth the investment unless, as I said above, you identify a critical requirement in your data discovery session that mandates it.
My 2 cents, hope it's helpful!
Best,
Kristin