This depends entirely on YOUR interests. Do you enjoy cardiac, the adrenaline rush of ICU, learning about complex hemodynamics and IV drip calculations, and too-numerous-to-count pathophysiologic conditions? IF the answer is yes, then go for it! Externships are usually great learning opportunities. The experience you could gain might prove invaluable.
Beware, however - some "extern" programs offer little more to the nursing student than just being a CNA. Is this a true preceptorship position? Investigate fully WHAT learning opportunities you will have in this position. Will you be allowed (for instance) to start IV's, administer medications, calculate pressor drips, set up and read hemodynamic monitors, take blood samples, etc? -All under the direct supervision of your preceptor RN on the unit, of course.
Also be aware that if this is a true preceptorship position, you will work the same hours as your preceptor - 12 hour shifts, days, nights, weekend, etc. This may be difficult for nursing students with families, small children, etc. You will need to plan ahead.
Best wishes to you in your decision