Memtest86+ v Memtest HCI
Neither is perfect , it's as well to understand the weaknesses.
Memtest86+
Strength:-
Tests nearly all memory as it runs in it's own environment, using much less
ram than windows

Wide variety of tests but :-
Weakness
Doesn't test memory as it is used in windows. Only tests 5 & 8 come close to emulating windows operation. This often means that you can pass this memtest86+ but still have issues in windows.
Memtest HCI
Strength:-
Runs in window so no no issues with emulation of windows memory usage, complements 86+ weakness.
Weakness
Only tests windows free memory - The full version will run outside the OS , like 86+, though in fairness it isn't that dear.
Only by running both for longish periods do you get comfort that memory is ok.
'86 is useful for a quick & dirty test when say trying to find optimum
ram voltage and timings. I use test#5 for this , start with a low voltage & keep upping voltage till test#5 runs ok. Test Timings by running test#8.
Running HCI for a while after '86 #5 & #8 have passed, is useful.
If memory has been stable for a while and you start having bsod's etc., I run HCI