Mei has all around better suiton feats.
She's shown;
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, which far outclasses tobirama's water wall. No discussion needed.
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, which also far outclasses that water attack tobirama did in part 1
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, which took zabuza 42 handseals to do an inferior version; Yet, she did it quick enough to combo with the other kage. Considering she used dragon flood, after water spout, without handseals, it's already implied she only needs 0 to 1 handseals for a 42 seal jutsu.
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, which appears to be on a level greater than zabuza.
Already, she has shown more suiton than tobirama at 4 - 3. Furthermore, In all those scans, though water spout and dragon bullet can't be confirmed, it could be seen that mei used 0-1 handseals to perform high level suiton. On top of that, she was in a near desert environment, with no water and possibly little in the atmosphere. Like you've already stated, she far outclasses tobirama in defence (hidden mist + water spout) but, she also outclasses him in offence (dragon flood + dragon bullet).
A misconception you made, is that suidanha has a better feat than dragon flood. Yea, suidanha cut the god tree, but everything else, including blunt weapons did that. But, because mei's suiton couldn't break a susano'o, it's seen as low class. Take a second to look at the jutsu. I'll break it down for you.
[
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][
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] - In that first scan, you'd notice that the dragon flood first envelops the target in a mass of water. Right there, we can see that an unprotected target will be rendered heavily injured by the crushing force of all of that water. As if that weren't enough, we move onto the second scan where we can see the dragon flood follows up with a sharp 'tongue'; An unprotected foe will be impaled with tremendous force and most likely killed on the spot. If you didn't think this jutsu packed a serious punch (especially that tongue), let me give you a demonstration. Danzo used
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, yet susano'o was able to completely tank the attack without falling or budging; However, mei's suiton was able to propel madara's susano'o through the air at high speed. That right there gives an idea of just how much force is put behind this crush-stab attack. Without a defence, the opponent will no doubt be killed on the spout. In fact, due to the size of the attack, an entire group of enemies can be killed in one shot. Now, tobirama's attack does have it's advantages, and is very powerful. As shown with ma, it has the potential to slice through concrete but, due to the size of mei's suiton, and the methods of dealing damage (crush + stab as opposed to simply slashing), I think it's pretty clear that mei's suiton is, by quite a considerable marjin, better.