Herbie -- usually a stock swap is structured as a "tax free" transaction -- not that tax will never be due, but that the swap will preserve the original buy date and basis from the acquired company's stock.
So if the deal were a simple 1:1 swap, and your AMRN basis was $3 per share, then you'd have a basis of $3 in PFE stock, with the same original buy date (for long or short term purposes).
Cash for stock deals -- it's treated like a sale upon receipt of cash. If you have the stock in an IRA (especially a Roth), or it's already long-term gains, then maybe cash works. If not, stock may be better.
Sometimes in these deals they let you elect which form to take, or even proportions of each.