I've been using AMPScript and I've seen the documentation, run a few tests, and I've done a bit of Googling, but off-hand, I'm not really seeing a notable difference.
Returns the current system date and time.
P1 Optional parameter. Enter 1 or true to indicate true to preserve the email sent time for post-send resolution of Now. Example
Given the date and time 10 a.m., March 1, 2008.
So it would appear you'll only see a noticeable difference if a sizeable amount of time passes between email sent time and when the code runs.
The static method
Datetime.now() has no parameters so you should not use any. Attempts will be ignored by the system or cause en error.
Returns the current Datetime based on a GMT calendar. Signature public static Datetime now()
Return Value Type: Datetime
The format of the returned datetime is: 'MM/DD/YYYY HH:MM PERIOD'
Example datetime myDateTime = datetime.now();
Now() will give current system date, If a user opens the e-mail again or views it as a webpage it gets updated.
Now(1) will always return the sent date. Example:
Sent: Email is sent on Jan, 1, 2017 10 AM
Open 1: User first opens the email at Jan, 1, 2017 18 AM
Open 2: User opens the email at Jan, 2, 2017 16 AM
Now() = Open 1: Jan, 1, 2017 18 AM Open 2: Jan, 2, 2017 16 AM Now(1) = Open 1: Jan, 1, 2017 10 AM Open 2: Jan, 1, 2017 10 AM