Classic 5-ID. First, a few comments about the Garrett Ace 250. Right now, dollar-for-dollar, the Ace 250 is one of the better values out there, and they are very easy to use, and in favorable ground they work pretty well. But the operator doesn't have any control over the Ground Balance adjustment and, from experience with all of the Garrett models, many will face some challenges in more mineralized and/or iron-trash environments.
I like the Ace 250's 4-AA power supply, preferring AA's to 9V batteries. I like the easily accessed controls. Heck, I even like the color. But I am not pleased with the too-short upper 'S' rod that could use more length between the armcup and grip. Also, Garrett's audio can have an annoying little raspiness sort of thing in high quality headphones. I do like them, or I wouldn't have owned two of them, but I prefer the proven Classic performance.
The Classic 5-ID is based on the time-proven Classic ID circuitry with just a little front-end revision to change the operating frequency and change the coil connector jack. The Classic 4 and Classic 5-ID are specially made for Kellyco by White's, and they operate at about 8.1 kHz. They share the same coils as the White's Prizm series, which right now is only the 8" & 950. I do believe you can order the Classic 5-ID with the 8" rather than the 950, and personally I would do so.
I enjoyed the Classic 5-ID when I did an early-release review of it (review at http://www.kellycodetectors.com/Field%20Test%20Reports/WhiteClassic5ID.htm), and two of me good detecting friends have a Classic 5-ID in their arsenal because not only does it provide the great Classic performance in iron trash ghost tons and such, but that way Chris doesn't have an interference problem with Aundrea's Classic ID or XLT or my IDX Pro or Classic III SL.
Both are very good units, but my preference is the Classic 5-ID because I know how well it performs at the trashy sites I like to hunt.
Monte