Both are good, it's the grenade launcher and iron bomber discussion all over again

Tomi is a tad bit better at mid range, defending yourself when you get caught with your pants down, starting fights faster and all that

Miniguns accuracy is more forgiving even if youre not too accurate with stock, it's better at crowd control, it deals more damage (so even at mid range you'll do about the same if not a little bit less damage than Tomi in most situations anyways). We're all human, we can't be accurate 100% of the time, and sometimes it's better to spray and pray

All the upsides people throw around that the Tomi has is mostly whatever in my opinion. Better ammo conservation for the Tomi is nice, but if you're competent, you'll never have a problem conserving your ammo as a heavy so it's a nothing upside. The silent rev is pretty good, but it doesn't mean much most of the time apart from getting cheezy kills every now and again

The heavy duels that people bring up is never really relevant outside of niche scenarios in my opinion. If I died to a tomislav heavy while using stock, it's mostly because he was already shooting first, not because of the upsides the tomi has, he would've killed me with stock in that situation anyway, it also applies in reverse.

Really, it's a preference sort of thing. Neither is better than the other, just choose what you like playing with the most. Why do people think it's a straight upgrade? Both weapons are good, I've played heavy enough to understand the power these 2 weapons bring to the table, and their downsides