I like ON GS because it tastes good, though I flavor hop every 5lbs, mixes well, is easy on my stomache, has an added amino blend, is sweetened with stevia and not aspartame, has a good grams/serving ratio, and is decently priced for the quality. I fell in love with it a few years ago and havent had a need to switch.
ATW gets great reviews on this board, though I have not tried it myself. Their isolate is cheaper than ON, but more than than the ATW blend. I like isolate because of the digestive issues. It does not, as far as I know, have added aminos, but it does have a better serving proportion for 5lbs than ON by a few.
Considering your emphasis on budget, I'd go with the ATW product you prefer. Plenty of folks on here like it just fine.