is there difference b/w incorporate &then amend docs to add investors vs.incorporate&do financing at the same time?

You can do either one although I believe the second alternative would be more efficient. If you do that bear in mind that a sole incorporator may be liable for the corporate debts until officers directors and shareholders are on board.

While it can be done either way, the two step process (incorporating first, then amending articles to add investors) is often a useful structure. Provided you do the entity formation early enough, you can put initial stock, option grants, etc. into the founding team's hands at a low price, before any outside financing event establishes an external valuation for the stock that makes providing inexpensive equity to the founder team more difficult (at least without tax consequence). Also there are typically some corporate basics that should be taken care of (assignments of founder intellectual property to the corporation for example) that require a corporate entity. You will look most professional to your investors when you have such business wrapped up before they do their diligence. Congrats on your progress and good luck!