Swipe routes the order to all the ecn's you specify to fill up to your order size. Blast sends your order size everywhere as seperate orders.
Say the stock is spreading up and I want to buy it. If i set my size to 1000 shares and swipe to a limit price, my order will route to all the market centers I have set for that key until my order is filled for 1000 shares.
If I do the same with the blast key, I am sending multiple 1000 share buy orders up to a limit, each going to the market centers programmed for that hotkey. I could be filled more than 1000 shares, but I use this key when I really want the fill and if I get more size, that's a bonus. Unless I'm wrong and get filled on everything, then that's almost always bad.
What key I use depends on the situation. The good hardcore scalpers I know send out lots of orders with lots of size and try to pick off good fills wherever they can. If they are right, they are in the money almost instantly and can use a super tight stop. Btw, I'm talking listed scalping here, not nazz, I'm not very familiar with scalping those stocks.