What you wear doesn't matter, but if looking nice puts you in a good mood id splurge on some fitness attire if I were you, look good, feel good gave more motivation,.
Running will obviously burn more calories per mile, but you will likely walk longer and might burn more. Its difficult to answer. Your best bet is to alternate cardio and strength training, it burns more than just plain cardio and tones while your at it.
You want to target the abs in many different ways ,,,, sit ups and crunches are fine but you also want to target the obliques and lower back also. So look up a workout called "squirms" "side sit ups", standing sit ups",plank position variations"...... Pilates will help strengthen core and teach you abdominal/ breathing to maximize your benefits. Yoga has certain positions that are excellent an toners because they use your body weight and improve your form, try " crow", boat ", "goddess, "and chaturanga push up".
But don't do anything too repetitively, switch it. Up and keep it fresh to obtain the most potential for change .