*If* I were to pre-order, where is the best place to do it? Online? B&M? Where?

If you want Open Beta and Early Access, then Gamestop or Best Buy. If you don't care about Early Access, then look over all the offers to see which you like the best.

There are few places offering pre-order incentives, such as Best Buy, Gamestop, and Amazon.  They each offer different things, I believe.

    If you go with Gamestop you might think about digital download. In theory that should allow you to get the game (in advance) and the key. You will also avoid any shortage issues that any 1 B&M store may have (no driving around looking if 1 store short orders). You wont have a physical copy but that doesnt matter much IMO since the game launcher and patcher should be available from your cryptic account page.

    That said it is Gamestop they are known for issues with customer service and some accuse them of  console bias and lack luster pc gamer support. I have not personally experienced anything other than a general lack of knowledge about MMOs and some pc games by the people working in the 1 store nearest to my location.

The options for bonus items are as follows :

@ Amazon.com 

Pre-order Bonus Items
Pre-order today and receive access to the beta, which begins on 8/17. Additionally you'll get three in-game goods (Harlequin Headpiece, Grenade Chest Strap, and Black Knight Helmet) and an additional five Cryptic Bucks. Offer good while supplies last. Must be sold and shipped by Amazon.com to be eligible. Limit one per customer. Beta code will be delivered via e-mail, the code to access the in-game items will be included in the game box.

@Gamestop.com

Reserve Champions Online and receive:

• Access to the early start program. Collect exclusive items, and join the early start weekend to get a 3 day jump on the action!
• GameStop exclusive Insesctoid Airfoil which flaps up to 90 times per second and is ideal for the high-flying hero.
• Level 0 Flight. GameStop customers can utilize the awesome power of flight right at the beginning of Champions Online – something that normally takes users 5 levels to earn!
• A 30-day free trial.
• Access to the Champions Online Beta starting 8/17 and ending 8/27.
• 5 Cryptic Bucks which can be used to purchase a variety of premium in-game goods.

Available for online and in-store customers only while supplies last.