Unfortunately, the difference in (radio) signal strength between the Medtronic and Dexcom CGMs is the most clear cut and unambiguous difference, at least based on my own & others anecdotal experience.
During my training session for the CGM one of the first things my Medtronic rep mentioned was that if your body is between the MiniLink transmitter and your pump, it can block the signal and result in first the “Weak Signal” alert and then about 40 minutes later the “Lost Signal” error. Because my rep warned me about this I assumed the other reps also did. But perhaps it only occurred to my rep to mention this because she also used the Medtronic CGM and had experienced this design flaw first hand. 
It’s just one of those things that those of us who use the Medtronic system learn to cope with, I guess. 
So it seems very possible to me that if your pump is allowed to “float” while you sleep that, depending on where your sensor was inserted, the pump could end up positioned so that the signal needs to pass through your body to reach the transmitter, which usually does not work. Whether or not you eventually end up with “Lost Signal” error when this happens depends on how much and to where your pump floats while you sleep.
In lieu of an actual solution, the work-around Medtronic has gone with to try to minimize the connection problem is to include a data buffer in their transmitters. The original/older MiniLink (introduced 2007) can buffer up to 8 readings (40 minutes). The newer Guardian 2 Link transmitter (introduced in 2014 along with the Minimed 640G pump and is only available outside the US at this time) can buffer up to 120 readings (10 hours). (Woo-yawn-hoo. Another example of Medtronic’s aggressively mediocre innovation IMO. 
)
The practical result of this is that if the pump loses its connection to the transmitter, but the connection can be re-established within the 40 minutes limit of the MiniLink’s buffer, then you will not get a “Lost Signal”. The buffered data is transferred to the pump and the system just keeps on rolling along. However, you will get the “Lost Signal” error if the connection is lost and is not restored before the transmitter overflows its buffer.
You might want to look through your pump’s “Sensor Alert History”. I expect in addition to “Weak Signal” followed by “Lost Signal” you will also find instances of “Weak Signal” where the connection was restored and the “Lost Signal” never occurred.
At any rate, as you have found out, once a “Lost Signal” does occur the pump will insist on a calibration with a meter BG (Blood Glucose) before it will continue to report SG (Sensor Glucose) values. So it is worth trying to figure out a way to prevent your pump from floating into a position where your body is blocking the signal … if that is in fact what may be happening to you.
Do you think the above may be what is causing your problem?
While your body can block the signal, the tape over the transmitter will not … unless you buy and use some horrid kind of metallic tape. 
What sort of things did the Medtronic HelpLine rep you talked with suggest?