That's a pretty tough call there.
The advantages of the X-Terra over the Ace, is an easier to read screen, the X-Terra's have nicer tones than the Ace. The small coil is better for covering ground than than sniper coil. you can also get DD coils for the 50. You also have a volume control that the Ace doesn't have.
The Advantages that the Ace has over the X-Terra's is that it's 100 grams lighter. With the large coil on, it weighs the same as the X-Terra does with it's standard coil on. the Ace has an absolute killer of a pinpoint feature that is far more accurate than the X-Terra and gives off a tonne more target information that the X-Terra's just can't match. The X-Terra's VCO is pretty good, but not in the same class as the Ace. The VCO really helps in the sizing of targets if they are shallow.
If you are looking for something a bit different than the Ace, then the X-Terra is a good choice. Out of all the light weight machines on the market, the X-Terra's and in particular the 70, have the most pleasant on the ear and eye plus high end features on the market. If you don't like it, then you can always sell it and get another Ace again down the track. No point if you die wondering which was best.
Mick Evans.