IMO after owning the Hydro and handling several Midos I don't think you will see the "notch up" you're looking for. With its wave dial, unique bezel and day/date complication, the hydro has nothing on your Mido style wise. My suggestion would be to keep the Mido, its a handsome solid watch.

 

I agree that it would not be a significant upgrade. Plus, that's a good looking Mido!

 

Why do you want the Longines? Are you unhappy/bored with the Mido? Are you just trying to move the whole collection upstream in quality?

 

That's a sweet Mido, wouldn't even worry about what's going on with Longines.

 

I'm not much into dive watches lol, but even I think that Mido is pretty sweet.

They hydroconquest gets a lot of love on the forums, but like others have said I'm not sure just how much of a step up it would be. I'd say, with that hard earned money, maybe it's time--if you're still itching for something new--to consider a watch of a different style. If you've saved up enough for a HC, what about--just thinking randomly here, lol, but maybe helpfully--a SARB033 or 035? They're beautiful pieces, represent a different style altogether from a dive watch, and would leave you with some funds left over.

These pics have them on leather bands but they come stock with bracelets. Anyway, they caught my eye in looking for pics:

 

I like Longines, I currently have two and just sold my Hydroconquest that I had for 18 months from new. However I never really felt that the Hydroconquest justified its price, its a nice watch, but the quality of finish is not quite there in my opinion. Unfortunately I have no experience of Mido, but I suspect you would be underwhelmed and most probably disappointed if you did 'upgrade' yours for a Hydoconquest. There are a lot more worthy contenders for your consideration.
If you want real value for money then I would recommend you take a look at the Christopher Ward Trident C60 Pro which new is roughly 25% cheaper (even more if you catch one of their sales) but 50% better in terms of quality than the Hydroconquest. Or alternatively consider stretching your budget and/or buying second-hand.

 

Thanks for the replies guys!

I think by using the term "upgrade" I gave the impression that I'm going to get rid of my Mido... which is not the case, I'm simply looking for a higher quality dive watch to split wrist time with whenever I'm in a dive watch mood.

But I would want something that is "better" to further evolve my collection, since for the most part every watch I have purchased has been of higher quality than the last.

 

I really like your Mido. I have an older Mido Ocean Star Commander Datoday that runs like a champ!

Unless you're dying for a change I'd keep it. But if you're looking for something different you might want to consider the Longines Legend Diver.