By far the most popular in our salon is the C curl, also the 0.30 thickness with the longer lashes (i.e. 12 or 14mm). To promote the more natural look for older women such as myself, I wear a combination of 10s and 12s, 0.20 thickness, J and C curl. It is down to personal preference but clients who have never tried them before and they leave themselves in our hands usually love the difference it can make and had we asked them prior to application, we find that they would have gone for a more conservative look. So find yourself a knowledgeable eyelash extension expert and follow her advice. Hope this helps.
The C or D curl are most popular with my clients. Both allow the lashes to be more visible when the eyes are open. The J curl is not as visible, depending upon the shape of the eyes. Also consider combining C, D, and J for an even more natural look
The majority of my clients prefer the more glamourous d curl with extensions but it's a personal preference. Those who are unsure if it will look too glam for them decide on a j curl, then at their next appt they want d curl.