I have an Aeroplan account (Air Canada), but am wondering if I should start a Mileage Plus account with United. Would that provide any benefits? If I wanted to get enough points for other international travel, what program should I join? Thanks in advance for your help!

(1) What is your home airport? (SFO, LAX, MCO, etc.)
San Francisco (SFO), Toronto (YYZ)

(2) What types of fares do you usually buy ? (C, F, Y, etc.)
Economy class, discount/special class

(3) How many miles do you usually fly each year? (<25K, 25k-50k, >50k)
<25,000

(4) Do you have any kind of status at present? What is it? (UA Premier, M&M Senator etc)
None

(5) What is most important to you in a FFP? (Frequent upgrades on travel, priority services when flying the airline, good award redemption rates, free lounge access, etc.)
Award travel, distant second: class upgrades

(6) Which routes do you fly most often (US Domestic, Transpacific etc)
SFO<->YYZ once or twice a year, Transpacific or Transatlantic once a year

(7) Preferred Airlines
To get back to Canada, I've only flown Air Canada, Continental or United (prefer AC the most). On my transpacific/transatlantic flights, I fly whatever's cheapest (thus, I have some points with BA and Korean Air).

(8) Do you travel for work or pleasure? (Can you chooose your airlines, class of service?)
Pleasure, can choose my airline

Thanks for your help!