Huh? I don't think EITHER of these are operational rules. You have to plan for enough fuel to fly to the first point of intended landing, go missed, go to your alternate, and still have 45 minutes.
Once you're airborne, I don't believe you have to refile your alternate if it goes below alternate mins (under 91 anyway), nor do you have to use the alternate you filed, nor do you have to land with 45 minutes of fuel in the tanks.
In other words, if you land with 10 minutes remaining because you held waiting for the weather to improve at your first intended landing point, rather than head for an alternate, you haven't violated any regs.
But I'm willing to be proven wrong, with the appropriate regulation citations.
§ 91.167 Fuel requirements for flight in IFR conditions.
http://ecfr.gpoaccess.gov/cgi/t/tex...&view=text&node=14:2.0.1.3.10&idno=14#PartTop (a) No person may
operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to—
(1) Complete the flight to the first airport of intended landing;
(2) Except as provided in paragraph (b) of this section, fly from that airport to the alternate airport; and
(3) Fly after that for 45 minutes at normal cruising speed or, for helicopters, fly after that for 30 minutes at normal cruising speed.
(b) Paragraph (a)(2) of this section does not apply if:
(1) Part 97 of this chapter prescribes a standard instrument approach procedure to, or a special instrument approach procedure has been issued by the Administrator to the operator for, the first airport of intended landing; and
(2) Appropriate weather reports or weather forecasts, or a combination of them, indicate the following:
(i)
For aircraft other than helicopters. For at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles.
(ii)
For helicopters. At the estimated time of arrival and for 1 hour after the estimated time of arrival, the ceiling will be at least 1,000 feet above the airport elevation, or at least 400 feet above the lowest applicable approach minima, whichever is higher, and the visibility will be at least 2 statute miles.
[Doc. No. 98-4390, 65 FR 3546, Jan. 21, 2000]