Which of the organisations is the bigger? The one you're currently working for, or the other?
I worked for a company that "merged" with another. We were the smaller business and in reality it was a takeover with the main business owners staying on only for a limited period of time while they handed everything over.
We all TUPE'd over to the new company and within a couple of years we'd all left as the bigger firm ethos and the poorly managed transition made it a horrible place to work.
We'd been given the option of redundancy when the merger went through as it also included a relocation, but it was an office in Central London, which I'd never done before, and I was in my mid-20's so I thought I'd give it a go. I lasted 4 months!
I think if I'd been in the bigger organisation in the first place there would have been little transition and little would have changed.
In summary, wait and see what happens OP. See how the merger progresses and get your CV brushed up in the meantime, just in case they either restructure, or you just don't like the new business.