I have used Dexcom for the past year, and prior to that used MM CGMS for 3+ years. I recently upgraded to MM Revel 523 pump, and wanted to test out the new features related to CGMS. I had an expired MM CGMS sensor, but decided to give it a try anyway.
For 6 days, I logged more Meter BGs than normal, and kept track of the Dexcom and MM sensor readings at the same time. Both were inserted on my stomach, to be as 'fair' as possible.
In general, my BG rises or falls gradually, and Dexcom usually shows the flat or slanted arrow, and rarely the single or double up/down arrows.
I was surprised the MM was 'closer' than I had remembered it being, prior to switching to Dexcom. But back then, I had been eating more 'white' carbs, and not pre-bolusing, so BG rises/falls were much faster. As a general rule, it makes sense that with the 20 minute lag, a 'flatter' trend would usually result in closer numbers.
I averaged the difference between the Meter BG and CGMS, as shown here.
Dexcom sensor ended up being very accurate after day 7, and I used it for 14 days total, a new record for me. Typically I use for 9-10 days. MM did not work at all on day 7, and ended with CAL ERRORs, and a very low ISIG.
So over the 6 days, the average difference between Meter BG and Dexcom was 13 points, while Meter BG and Minimed was 21.5 points. I did 12-15 Meter BGs/day, and did calibrations 3/day at the same time, with same number for both. All calibrations done when 'stable'.
And here are a couple comparisons.
In pic below, meter BG was 163, after a meal, so reasonable that Dex was showing a lag, and did climb to 160 within 20 min. But MM overshot the rise to about 180, and have no clue why it then dove to 95.
It seemed like Dexcom was better at catching the up/down trend sooner, and matched my expectations (based on bolus, food, exercise), and confirmed by meter BGs. So in general, I had more confidence in the trend information on Dexcom.