Answer: First, congratulations on your offer - as any candidate in the current job market can attest, it is still a very competitive market in a depressed economy, so having a job offer in hand is not something to be taken lightly.
The relative ease of moving from a law firm to government, in-house, or alternative service/consulting firm work and vice-versa is highly dependent on the practice area. As a general rule of thumb, most law firms have a strong bias towards candidates who are coming from a current position in another law firm, and preferably an AmLaw 100 firm or a prestigious boutique. There are a few notable exceptions, such as securities and white collar litigation candidates who have enforcement-side experience in government investigations from the DOJ or SEC, or patent attorneys and agents with strong technical backgrounds and substantive prosecution experience at tech companies and research labs.
In the tax world, however, the bias against candidates who are not coming from a large law firm unfortunately remains, and even top-level experience in sophisticated tax consulting at an accounting firm is not valued as highly. In fact, there are some top firms who have a policy of not considering lateral tax attorney candidates who do not have existing big firm experience.
Thus, in terms of whether to take the accounting firm position, the decision really depends on your ultimate career goals. If you know you want to practice within a law firm, and you do not immediately need a job, you may want to hold out for a law firm position. However, if you need a job, want a stable long-term practice, and do not mind the potential impact on your ability to lateral to a law firm down the road, you should seriously consider the accounting firm's offer.
These things always vary by practice area and market, so if you want to get some expert perspective on your short- and long-term career options, talking to a good recruiter is a great place to start.