The B737-800 is more like tomorrows news than yesterdays news! Take the command, (if you are not too late), a command on anything is better than the RHS on anything, in the long term. What fleet can you expect to move to after the A320/B737? Big Airbus or Big Boeing?
With 5000 hours on the A320 you will be expected to do a trouble free command course, when going onto the B737 you can reasonably expect some latitude in your handling until you have adjusted and that shouldn't take you long, you will be pleasantly surprised!