Which treatment could help me? Velashape or Thermage/Titan? (Photo)

In my opinion VelaShape 3 is superior to Thermage. The  advantage is their technology. Today, there are no other methods available that include a combination of Bi-Polar RF, Infrared Light, Vacuum and Mechanical Massage like VelaShape™, and no other method can guarantee the clinically proven results, efficacy and safety in 4 treatments as the VelaShape™. VelaShape™ is the only non-surgical medical device cleared by the FDA for circumferential reduction.

You may be a good candidate for the combination Ultrashape and Velashape III, spaced a week apart for approximately 12 sessions. If the scar is below your bikini line, this may be the non invasive way to go. Alternatively, you may seek a surgical opinion for scar revision as well as some type of abdominoplasty depending on the degree of skin laxity. 

Thank you for posting a photo. I recommend
learning more about V-Shape; this new laser will accelerate your metabolism for
releasing fat while tightening the skin and improving its texture. Consult with
your doctor to decide which procedure is best for you.

The picture you provide represents a wide scar and wrinkling of lower abdomen due to skin excess. You have had many changes, including rapid weight gain requiring abdominoplasty and subsequent pregnancy, C-section and further weight loss. Given that you are a "bikini competitor" you may want to consider scar revision surgery. The scar is too wide and the skin too wrinkles for any non-surgical treatment to be effective and lasting in a meaningful way.