If price is your primary consideration, you can buy the Ace 250 for around $200. A new X-30 will cost around twice that much. And a new X-50 will be another $150 higher than the X-30. Based on what your budget allows, if you have never owned or used a metal detector before, you may want to go with a new Ace 250 or a used X-Terra. I know that the X-Terra is a far "better" detector for many of us. But I am speaking of folks who are not new to the hobby. Folks who already know whether they enjoy swinging the coil or not. Folks who are willing to do the research, dig the holes and spend the money to get a detector that fits their needs. If you or I had a dollar for every detector leaning against a closet wall somewhere........ most purchased with every intention of using it on a regular basis.......... only to find out there isn't enough time. Kids have little league, wife wants to go bowling, you spent too much on that bass boat and feel like you should be fishing.......... well, we'd both be rich! Life only gives us 7 days each week, with 24 hours in a day. There is only so much time to spend doing what we really like to do. If detecting isn't going to be your forte, or you aren't going to have enough time to dedicate yourself for successful hunts, you'd be better off finding that out with a $200 Ace than a $400 X-30 or a $550 X-50. On the other hand, if you are 100% positive that you are willing to learn the proper technques, do the research and learn the detector, then don't buy something that will dig every pull tab and rusty nail, and quickly turn you off on the hobby. If you are going to hunt clad in the park, any decent discriminator will do the job. For the beach, make sure the coil is waterproof. If you are wanting an "all around" detector that will go deep, is sensitive to small targets, has a wide variety of coils available, offers excellent notch discrimination characteristics and offers the user the ability to fine tune the detector for the site, the X-Terras will prevail. If your budget will allow, of the three detectors you mentioned, I'd opt for the X-50. In addition to the features offered by the X-30, the X-50 also has an adjustible Ground Balance, "tighter" notches than either the X-30 or the Ace 250, 4 tones opposed to three, two programmable Patterns plus an all metal mode, Noise Cancel channels, more refined settings on volume and sensitivity AND the X-50 allows you to use all the coils available in both 7.5 kHz or 18.75 kHz. For a more in-depth comparison between the X-30 and X-50, go to the FAQFAQ section at the top of these posts. In there you will find links that take you to the Operators manuals for each. Since the manual for the X-30 and X-50 are in the same book, you can read about the functionality of each, and determine whether or not the price difference is something you can justify.
Now with all that said, I am not putting down the Ace 250. For the price, it may do all you need to do. It simply depends on what it is you are going to be hunting for and how serious you are about the hobby. Having owned all those mentioned, and having been blessed to be in this hobby for 36 years, for the type of hunting I do, I'd tget an X-Terra". But that is simply my opinion.
If you have any specific questions, fire away. Lot of experienced hobbyists on this forum. And most are willing to take the time to give a hand. Best of luck with which ever detector you chose. HH Randy