When the shopping trip has a clear objective, I prefer to shop alone.
For example, grocery shopping has a clear objective. I can look through the coupons and store deals, plan meals for the next few days, note things that I'm running short on, use that info to make a shopping list, go through the store isle by isle to get non-refridgerated items, go around the walls to get refridgerated items, and buy nothing else. I accomplish the objective in the most thorough and thrifty way possible given a set amount of time.
When other people get involved, chaos follows. They whine about me spending time to look through coupons and store deals, whine to stop me from planning meals, whine to stop me from making a shopping list, run frantically from one side of the story to the other, throw items into the cart that I would not have put on my shopping list, and whine to stop me from buying in bulk, so I end up paying significantly more than I would have otherwise, know that I am going to need to throw away a lot of that food (and money), get home to find that I do not have enough room to store everything, throw everything into disarray to make a place to store things, and then find out that I need to go back to the store to get something I forgot. (sigh)
When the shopping trip does not have a clear objective or I can benefit from hearing peoples' subjective opinions -- such as when I go shopping for shoes and clothes ("Do these jeans make my ass look good?") -- then I prefer not to shop alone, but it's not a big deal to shop alone.