I have both sets and both are in my top 2 for chifi DD under $100 (for my personal tastes I would put the Diamond as #1 and Ninetails as #2).
Would agree with
@loomisjohnson that ninetails is better for your preferences cause of the tuning filters to suit different genres and wider soundstage. Diamond is more smooth and laidback.
Both sets are typical of DD tuning, with timbre/tonality/coherency being their forte (maybe Diamond has better timbre than the Ninetails). Both have quite good technicalities for a DD set but won't beat similarly priced multi BA sets in this area (Diamond's technicalities are still better than Ninetails except in the area of soundstage). Both can hit in the midbass quite hard (ninetails with bass filters on). In fact with a good source, ninetails can almost hit basshead levels with both bass filters on and give jawrattling bass without EQ.
I don't listen to metal but i would think a multi BA/hybrid would be an advantage over these two DDs in terms of technical excellence for metal and genres with lots of complex instrumentation and fast passages/synthetic instruments. These two are best suited for accurate representation of acoustic instruments and vocals. But nevertheless i would say for more hardhitting stuff, the Ninetails would still be the better option for you.
Ninetails looks weird but it is comfortable as slater says. Isolation is subpar though, so not the best for on the go use. And the tuning filters really work and are not gimicks unlike some other chifi.
And you're in luck, there's an aliexpress sale going on now. Lowest I've seen the ninetails go last year was about $75 usd after coupons, good luck!