If I were you I would save my money on the activia if that is the only reason you are choosing that brand.
All good yogurts have live cultures, and more than enough probiotics to satisfy your desire for gut health. Activia has just been marketed more to the masses, especially toward women/housewives/mothers, who do most of the grocery shopping.
There are 3 strains of probiotics, activia just happens to have one that one of the strains a lot of other brands don't use. That doesn't make it better. It's all marketing.
You must like yogurt because you're eating a quart at a time. You might want to explore other brands. Prices can vary widely, but so does the taste, and satisfaction level. Experiment.
Also, I wouldn't get all excited about whether it is low fat or not. This is a whole different subject, but the fact is, fat is good for you. It's what our bodies were meant to burn. Even if you can't wrap your mind around that (it's a big subject), you are certainly better off getting a full fat yogurt that is not full of sugars, than one that is low fat, but loaded with sugar.
I personally don't eat yogurt often, as I have problems with dairy. However, there are some really really good ones out that that will fill your probiotic goals.
I buy yogurt for my husband sometimes, and I've tasted them. Our personal favorites are in order of taste, are Noosa, Brown Cow, and a brand that is goat yogurt, Redwood Hill Farms. That last one is more expensive, but hey, goat yogurt is just more expensive.
Another reason for using full fat yogurt is that you probably won't find yourself wanting to eat an entire quart, because you will be more satisfied, satiated, because of the fat.
Also, if your eating that much yogurt, it's cheaper, and less wasteful as far as packaging, to try brands that also sell in quart sizes.
Bottom line, just make sure the yogurt has live cultures.
Regarding probiotics, be aware yogurt is not the only excellent source.
Items like raw saurkraut (not the crap that sits on the shelf, the kind you get in the cold section of the store, or you can make your own), kimchi, kefir (it's like a yogurt drink) can mix it up for you.
Putting a dash of apple cider vinegar on your salads, in your soups, whatever, will also serve the purpose.
Re saurkraut, I personally like it with breakfast. It is really good putting a small amount on top of your eggs, avocado, bacon, etc.
Breakfast is, as far as my experience, the quickest and easiest meal to get in a ton of nutrition. My typical breakfast (which I may eat at 10am or 10pm
) gets in 4 or 5 vegetables, my probiotics, protein, and fats. I get a lot of bang for my buck out of breakfast.