Flakes are better, but still recommend the pellets.
When you're at the store, look at the price per OZ. For retail locations when they're NOT on sale OSI flakes are roughly $5.50/oz & OSI pellets are $3.80/oz.
Flakes & worms are not good because they're pretty much filled with air which leads to bloating. When worms are dried out the most beneficial nutrients are gone.
Also for flakes ONCE exposed to water, the vitamins & nutrients disperse. If your fish don't eat them IMMEDIATELY within 1-2 seconds, they lose nutritional value the longer the float.
Pellets are better @ keeping the nutrients inside till the fish consumes it.
From your photos & the 1.5" length of your bettas, if they were mine to care for...... I would NOT feed them till 5-7 days from now in hopes their bellies would slim down a bit.
It's totally up to you what type of food to feed. But I would HIGHLY suggest pellets instead.
NLS is the brand I would recommend based on the company's reputation, the CEO, and all the people in the labs who've spent years researching/developing & improving their product. NLS is #1 in my opinion for LOCAL retail foods.
OSI is a good company as well, but they don't really answer questions & their site needs updating. You can't even contact OSI though their email because the link is BROKEN.
Kithy, do you have the Thera-A formula?