M&A Advisory and Valuation at a Public Accounting firm: How difficult is it to move to Investment Banking

I recently received a FT offer for the Mergers & Acquisition Advisory and Valuation group of a Public Accounting firm. They are around the Top 10 mark in the U.S. in terms of size.

My background: Senior at a Non-Target with no IB experience. Investment Banking is my first choice, however I realize that I am facing an up hill battle especially considering how late it is in the recruiting cycle. I have several weeks before I have to respond to this offer and am waiting to hear back from some from smaller IB firms.

Not sure if this is all obvious, but thought it may be helpful to provide specific information as to what i would be working on. The company has a large number of Private Equity clients for which they help identify potential acquisitions, and do valuation on the target company. Additionally, they do post closing due diligence such as purchase price allocation. They provide much of the same services to their strategic buyer clients.
For sell side engagements, they occasionally run the entire deal, but also may only help prep the business for sale and prepare the offering memorandum and other marketing documents.

I am curious as to how difficult it would be to lateral to a MM or Boutique investment banking firm. Ideally, I would work at this firm for 1-2 years an then make the transition. My initial thoughts is that this is a good alternative to IB right out of undergrad where I can gain relative experience that would hopefully allow me to move up in several years.

What do you guys think?
Thanks in advance.