SuttaCentral
Lol. My Pali is probably better than my Textual Buddhism Chinese. There will be a Pali bias on account of that. But given its apparent polysemy in Pali, I would render the Chinese differently according to context.
I mean, who’s to say that the MA and SA translators were not binding themselves to one rendering of each word, despite the context?
sigh… but yes, Mun-keat comes to a similar conclusion after quite a detailed chapter in “The Fundamental Teachings of Early Buddhism”:
This comparison of the Dhatu Samyutta of SN and the Jie Xiangying of SA has revealed that dhatu is used to mean various different things in different contexts, including: “set of conditions responsible for initiating the chain of conditioned arising”, “natural individual characteristic or ethical quality”, “meditative attainment”, and “material element”. In general, the common shared meaning component of the term dhatu can be identified as “natural condition”.
Whereas the gist of 界 in the Chinese Buddhist Dictionary is I guess ‘unit’.
And are the arupa-dhatu and arupa-ayatana the same character of 界 ?
Haven’t gone that far. Already exhausted just trawling through SN 14.
BUT, I’ll titillate you with this -
無量空入處界 = ākāsānañcāyatanadhātu. SN 14.11 
pft, I don’t get shocked any more, just 
but ākāsānañcāyatanadhātu is actually one of (many better) examples that to me show that dhatu means characteristic or property
I translated this a while ago as
[bahuśrutāḥ are altogether] mutually ranked, mutually congregated, mutually responding, & mutually functioning
Thoughts? IMO if anything is serving as dhātu here it is possibly 類.
界 is much much more common, particularly in the literature that I presume you will be most interested in looking at. Like I mentioned before, I think it is more-or-less ‘border’ in the most general sense you can possibly think of the notion. It gets it’s semantic root from 田, ‘field’. 性 is closer to the kind of word that makes Buddhists queasy if there is no caveat on it. It’s semantic root is 心 appearing as忄, ‘heart’, but also meaning ‘citta’ or ‘mind’ in a Buddhist context (which is irrelevant, though, to its etymology), and it seems to have more of that “inner essence” kind of meaning, if I can speak as vaguely as possible. Keep in mind that an expert may have more refined opinions and might discount mine.
But in relation to your OP
Neither word is particularly less material or particularly less ontological unless the person saying it themselves have a certain philosophical orientation, IMO.
Again, the general meaning of dhātu across Buddha’s time Indian philosophy - as stated above, in the late RV., contemporary Upaniṣads, and even in late Br. and Up. - has a strong “essential” connotation of “desire”; that makes it a ground, then a field of external experience on a particular object (water,…, space,… etc.)
Now, as far as the “substance” this word (dhātu) can take, through its contextual forms in the suttas, it seems that we all agree that it has to do with “property”, (aka quality). The essential one being desire; and the substantial ones being moist (water), heat (fire), etc.
Let’s take the four basics material elements to simplify.
Water, fire, earth and air, are not just mahābhūtāna rūpa. They are also dhātus. They are the ground of something to be desired. And their substantial qualities convey that desire (and its perplexity). Then comes the field of experience itself (āyatana).
Usually, that experience is accompanied by sañña (whose correct translation should be “inquiry & assumption” - the same way that vedanā should be correctly translated as a “felt experience”.)
Then we understand better what dhātu means, when reading AN 11.9, AN 10.26, etc.
“Here, Sandha, for an excellent thoroughbred person, the inquiriy and assumptions on earth have disappeared in relation to earth, the inquiriy and assumptions on water has disappeared in relation to water, the inquiriy and assumptions on fire…,
…
Meditating in such a way, an excellent thoroughbred person does not meditate in dependence on earth, etc…
AN 11.9Householder, you should train thus: ‘I will not cling, to the earth element…, I will not cling to the water element…, I will not cling to the fire element…
MN 143The ultimate desire in and for a dhātu is in the kasiṇa.:
Some ascetics and brahmins, for whom the attainment of the water kasiṇa … the fire kasiṇa … is supreme, generated it as their goal.
…
By seeing the beginning, the danger, and the escape, and by seeing the knowledge and vision of the path and the non-path, the Blessed one knew the attainment of the goal, the peace of the heart.
AN 10.26Bhikkhus, the arising, continuation, production, and manifestation of the earth element … of the water element …, … of the consciousness element is the arising of suffering.
SN 26.9
One understands the “final cause”. Viz. the end of the “grounded” desire.
What’s in between (viz. the knowledge of what makes up the particular element; of its secondary qualities), is just substantial.
It’s probably workable, but I do have a few reservations -
rendering 相 as “mutually” is certainly permissible in modern Chinese, but I’m not aware of it having any adverbial usage in Middle Chinese or Buddhist Chinese where it appears to have only a nominal frame.
many of these Chinese translations appear to only be rendering the Indic nominal stems, without any attempt to render the grammatical case. Why this is so is debatable - piety or unfamiliarity with Prakrit grammar?
But, on the assumption that -
(i) the Indic text for these Chinese translations had a properly declined dhātu cognate in the ablative case as the cause/basis;
(ii) the coming together as a consequence, and
(iii) the same word order as the Pali,
I think the Chinese syntax would invariably put the cause first, before the sequel. That’s why I read the 相 as occupying the place of dhātu. Dhātuso might not have been recognisable as an ablatival to the translator, which led them to just render it as if it were merely a lemma without furnishing a preposition.