Invited to write a review on a field that neither my Prof nor I are experts in?
Your prof is thinking long term. You are thinking short term. Both of those are appropriate. But don't think too short term. Within a year you will want your horizon to expand and both the good opinion of your advisor and a publication will help. 

But, I didn't read you as implying that this must become the first priority over a period of time. I don't see why your experiments need to be put on hold while you read papers (probably fewer than countless). 

But, if your advisor wants to do this as a joint project ("write a review with him"), then you should be able to manage it as long as it isn't all dumped on you. I suggest you set a schedule to work together on it and lay out a plan. 

And maybe the new field will prove interesting and even valuable in your future. 

Of all these things, I think the good opinion of your advisor is the most important to manage. If you can do that somehow other than agreeing, then there is little downside. But there might be upside to agreeing. Your call, of course.