Not sure if this belongs in ATC or the Weather section. My question is - Most times the METAR for my airport doesn’t even closely match what I am getting from ATIS. Any idea which one (if either) is accurate for actual current weather conditions?
IRL ATIS is aviation and airport specific containing information not in METARS. For example, the VFR or IFR condition, birds or wildlife in the vicinity, etc. ATIS information may have local PIREP information such as wind shear or cloud tops. ATIS information may be out-of-date which is why tower controllers give current wind conditions to arriving pilots. METARS are used by both NOAA weather and aviation weather and contain information used for both weather reporting and forecasting. ATIS doesn’t report report long term trends like what is contained in the TAF. Pilots should be able to understand and use all the weather reporting tools available.
So if I need current winds and QNH to set my takeoff performance, which data should I use then?
Use ATIS when planning in the cockpit. This is the most actual infotmation vs. METAR in your OPS folder.
BTW, in my flying days METAR was treated as a “two hours TAF” - hence you can sonetime see a trend note at the end of a METAR message.
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The data in the ATIS should represent the real conditions fairly well. In the US ATIS is updated every hour by tower controllers, but if the weather is changing rapidly, they will record special updates as necessary, and warn pilots about them as part of initial radio communication.
If you experience significant difference between ATIS and METAR, something is wrong. Maybe one is randomly generated and the other is supposed to represent real world conditions. Or it may be just a bug in the sim. 
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ATIS often uses the same automated (AWOS/ASOS) stations that generate the METARs, however those reports themselves are often augmented or edited by observers. If your METAR doesn’t say “AUTO” after the date/time stamp, it’s been augmented by a human.
ATIS has to be updated any time official weather has been received. METAR, by definition is routine (usually around +55), but certain conditions can trigger a SPECI, which will cause an ATIS update. Note that real-world ASOS will radio-broadcast updated, to-the-minute weather to pilots, but only the hourly routine observations and SPECI make it to the published, official weather.
ATIS also has to be updated when braking action reports are worse than the last broadcast, or when changing runways, approaches in use, etc.
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