Hi
@emily deacon . Having read another post from you it appears you are looking for something to inform you when you're sugars maybe going low. Correct me if I'm wrong
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Dexcom is to the best of my knowledge more expensive to use.
I actually have no experience of using Libre, only experience of using Dexcom G4.
Dexcom initially set me back £800 to set up, and is currently costing around £ 60 per month to use, dependent on how long sensors last. Initial outlay for G5 is cheaper as you have no need to purchase a receiver ( £350 ) you just connect up to a smart phone. Sensors still cost £200 for 4 whichever option you choose. Transmitter (£270 ) is a must. Apparently people report that transmitter for G4 can last over a year but G5 transmitters don't last as long.
The libre system in its basic form offers no alarms and needs to be scanned to gain results whereas the dexcom constantly sends information without any input from the user.
In order for the libre to alarm and monitor continuously you would need to modify xdrip+ I believe. The go to guy for this is
@Scott-C he would be able to give all the information required regarding using / modifying the libre system.
Hope this helps. Good luck.