Are they the same thing because the have same moves and type but I’m just wondering if there stats are the same
They are the same except for the element. Omega is holy (in HI it was element less) and Tia is earth.
They are the same element in NeoMonsters.
Omegadragons stats:
HP: 4216
ATK: 5725
DEF: 3390
SPD: 38%
Tiamazus stats:
HP: 4128
ATK: 5605
DEF: 3315
SPD: 38%
Glad I have my Omega.
The stat difference is pretty much negligible, I tend to use my Tiamazus more as it has a +1 
My omegasdragon has +1
Basically tiamad is a slightly weaker omega
I have both now anyway…
And flarevern is a stronger version of omega
stats wise yes, but omega is definitely the better mon just because of its element
why is holy considered a better element?
I think it’s cuz there isn’t an element that Holy attacks are less effective against…could be wrong.
Your right
Shadow hurts holy
It all depends on your team for what one you should use
Offensively speaking, holy and shadow are generally better compared to the other 4 (fire water electric earth) because holy and shadow are not resisted by anything. Each one of the 6 elements hit super effectively against 1 of the other elements and thus each element is weak to 1 other element. But, the differentiating factor is that the 4 elements are not very effective against 1 element each while holy and shadow are not which makes holy and shadow better elements on offensive mons.
Defensively speaking, generally the 4 elements are better because holy and shadow do not resist anything while the 4 elements each resist 1 of the other elements.
Of course, there are exceptions to this (hence the reason I said “generally”). For example if you’re making an assisted element team (assuming it’s 1 of the 4 elements) then that element would be better than even holy or shadow for offensive purposes.
In the case of legendary poison eaters, the best 2 are omega and flarevern. Omega is generally the better mon to use but flarevern is particularly useful to counter a specific last biter: dragaia. Poison eaters are generally counters for hold ground mons, including the ones with last bite. Flarevern is especially useful in this case because he is the only poison eater that can take a last bite from dragaia (not godfeather though) and kill it back with a super effective poison eater (dragaia will live with hold ground then die from poison damage). No other legendary poison eater can survive a last bite from dragaia, so in a way you could say flarevern is a hard counter for dragaia (in case you haven’t noticed there’s a cycle here: dragaia beats godfeather, godfeather beats flarevern, flarevern beats dragaia).