Weather observations from METAR and SYNOP have the same accuracy.
METARs are half-hourly or hourly observations.
SYNOPs are observations made in a 3-hours basis (00 UTC, 03 UTC, 06 UTC etc).
Example: SYNOP at 12 UTC shows temperature 20C.
Next SYNOP at 15 UTC shows 22C.
METARS (for the same time period) may show:
METAR 12 UTC, 20C
METAR 13 UTC, 23C
METAR 14 UTC, 24C
METAR 15 UTC, 22C.