Well...its all down to the variety. In a work experience placement you want exposure to a great number of different procedures and activities.
Cardiology has, in terms of investigations, the ECG, Echos, Treadmill and Holters. You also have a invasive procedures. For example, cathaterisations, angioplasties and angiograms, pacemaker installations etc. Seeing so many different things is a huge plus point.
This is of course, on top of the more mundane ward rounds and clinics offered by all departments. Also Cardiac Wards are usually much more interesting than others - as they often command the greatest number of SHOs and Junior Doctors on-call at the same time (or at least, where I was). Someone will always be there to take care of you. The number of bedside procedures are also greater in Cardiac Wards. (I find) eg. the number of bedside tracheostomies done.
And finally, the curative aspect is much more rapid and appreciable. For example, if you see a primary angioplasty (where a man or woman having a heart attack has his/her coronary arteries ballooned in front of you) you will really be quite taken back by it all. The precision of the doctor, under pressure and the recovery the patient makes after the procedure is over is quite amazing. It is definitely worth-while watching and is something you might even talk about on your personal statement.