You will get a hundred different answers from a hundred different people. Just as I might say Ortho is exciting and Vascular is horrible, there will be someone else who feels just the opposite.
Choosing a field is much more complicated than just how exciting and enjoyable it is (although obviously thats a big one). Besides enjoying the surgeries, do you like the people who have chosen that field, how is the lifestyle (ie, if you have a family and hate emergencies, Vascular or CT surgery might not be the best choice), etc.
You're right - you can't shadow every field, but when you rotate on Surgery next year you will get a feel for some of the above. Unfortunately, there is no way to really know what Urology, ENT, CT Surgery, etc is like without doing it. If you find that during your basic science years you were most interested in the cardiovascular system and hated genitourinary, that might lead you in the direction of CT Surgery vs Urology. You may even find that Surgery AT ALL isn't for you once you've done your clinical rotations. Another completely different field of medicine may strike your fancy.
Read about the different fields, talk to as many practitioners as you can and keep an open mind about all of medicine, not just surgery.
