I know that grass fed is much superior, but I am on a budget in California. I used to eat liberal amounts of Kerrygold and Smjor but went down to getting Trader Joe's Organic butter. Even then butter is pretty expensive for me.
I am thinking of eating the organic but grain fed butter liberally just like how I did with Kerrygold, but was wondering if there was any downside to it.
I think then you'll want to make sure you have another source of omega 3 (e.g., eat some fatty fish). Also, I think you'll be missing out on CLA and vitamin A, but if you eat some pastured beef, you could get it from there.
Ruminant products are low in PUFA. You might miss out on a little omega-3, but less than 1% of the fat is going to be omega-3 anyway. Less conjugated linoleic acid - an uncommon fat. The bigger difference is going to be in the vitamin profile, particularly lower vitamin A and K.
The biggest difference is flavor, grass-fed butter tastes stronger, and more buttery than conventional butter.
Think of fat cells as natures toxic waste dump, all the bad stuff gets stored there. This is why most people feel like crap for a while when they are detoxing. I am not sure if it works the same with eating butter as it does with eating other types of animal fat, but you will be injesting the same chemicals the animal did, just to be safe I would avoid it. If you are on a budget you can get organic expeller-pressed coconut oil at walmart for about 5 bucks, and huge containers of organic olive oil at costco, nobody said you HAD to eat butter.