Have you seen the video on this thread? https://forums.brianenos.com/topic/285018-red-lnl-ap-vs-blue-xl750/
Here is my $.02.
Short Version: I have the Hornady LNL. I have never used a Dillon. If I was going to stick to a strict $600 or less budget I would get the LNL every time. If I was willing to spend a little more I might would consider the 750.
Long Version to support the short above. You might only be shooting 600 rounds a month now and you might be ok with basic and simple to start with. But, you have already said if you end up $1500 into it eventually then so be it. I assume this means you want the flexibility for the system to grow. I think you need a bare minimum of at least 5 stations on the press to grow without giving up too much. I think the LNL and the 750 are the same class of machine, but you can get the LNL for cheaper than the 550. The 550 only has 4 stations as well, so really it would be between the LNL and the 750 for me.
When I first setup my LNL, I manually fed cases and bullets and visually checked powder. I have since added a case feeder, powder check die, and I 3d printed a bullet feeder. With 5 stations I have 1 de-cap and size, 2 prime, expand/bell and powder, 3 powder check, 4 bullet drop, 5 seat and crimp. This allows me to load at a pretty good pace safely. Once I got the timing and indexing dialed in, it has been solid. If you have spent some time on the forums, all presses are going to take some time to dial in...even the $3K Mark 7. To switch over to load another caliber is about as fast and easy as it gets. The LNL has been a solid press for me.
But, I cannot deny the following Dillon has. Dillon is the defacto standard. The after market for Dillon is much better. The user base is much bigger. If I were in the market again, and was willing to spend a little more, I would give the 750 a serious consideration, but I could not tell you that is because I think the 750 is that much better or because I just want to taste the Dillon kool aid!