In general, arm cuff monitors are more accurate than wrist monitors.
You need to remember this:
Blood pressure is affected by gravity. If you are standing and measuring your blood pressure at ankle level, you are going to see some amazing numbers (just try), probably 220/180 or so. The explaination is fairly simple:
1 mmHg (millimeter of mercury) is equivalent to approx. 0,75 cm of water (as the density of mercury is approx. 13 times as high as water). Blood has about the same density as water, so in other words, for each cm you keep your arm (wrist) below heart level, your blood pressure monitoring will increase with 0,75 mmHg systolic AND diastolic.
If your arm is hanging straight down when measuring, you will get a wrong monitoring with your arm cuff but a severely wrong monitoring with your wrist monitor.
If you are going to measure your blood pressure, here is how to do it:
Supine: Support your arm on a small pillow so it is leveled with your heart.
Sitting: Support your arm on a table or similar.
Make sure you do not bend your elbow so much, as this may also produce wrong numbers.
P.S. I get 2 TOTALLY different reading on the wrist monitor when I lie down flat on my back and have the monitor over my heart vs. sitting up and holding it up at heart (which is what the directions call for). I've read that lying down give you a MORE accurate reading though. Very confused.
(It's always lower when lying dow).