Is it really "BETA"? Have you beat it to death in ALPHA / in-house testing ?
I like the #3 option -- a link on the established site to the new one.
Either way, I think it depends on the overall amount of traffic, the total number of products, product options, categories, payment and shipping methods, and if you handle multiple currencies, have assorted customer types, run any affiliate programs, need to test CRM ticketing system, promo codes, etc..
If it's not a small simple store, then I'd run both sites for some time -- leave the older established site up as www.example.com and run the new one as store2.example.com, (or some other domain or sub-domain). That makes it easy to temporarily close the new store if there is a critical problem.
I'd invite established customers to use the new store first, (ones you can count on for good feedback -- ones that know how the old store worked and can tell you what they do or don't like about the new one).
Even if you get all the products, prices, images, etc.. all looking good, you need to find out if order processing, inventory control and any CRM, marketing or reporting functions are working.. etc.. etc..
It can take a while to fine tune a large ecommerce site. Since money is involved, I'm sure you wouldn't want to find out you didn't test enough and are losing money, rankings, customers or anything else.
You asked, so that's my $0.02