Personally for my wedding, we bought the suits from Matalan... made a day of it by having lunch with the groomsmen in London and then going to try on and buy the suits. Initially I was going to do like many brides and rent the suits, and yes, budget was a factor. But then I thought 'well we have to buy the groomsmen a gift for the day, so why not a lovely new suit with a good memory?' so that was the eventual thing we did.
Obviously it depends on how many groomsmen you have (we had 2 plus my husband - though he needed a new suit anyway), as to whether buying or renting would be the best option.
If you do rent, and you have quite a few people t rent for, why not ask the shop if they could throw in something like free ties or matching cravats, just to ease the cost. For my husband we rented his cravat and wasitcoat, for the best men and hubby we bought the ties.
I guess at the end of the day, neither is a better option than the other. Although at least with a suit you buy it doesn't matter how dirty it gets. However, it will most likely come down to cost and budget. For many brides planning a wedding, sometimes it just costs too much, so they ask the wedding party to buy their own dresses and suits. If it's really relaxed, they wear what they have.
Do what suits you.
(had to squeeze in a silly pun didn't I? ;)