(i) "Phases" are a railfan category, and usually are defined by superficial (but visible) design changes the locomotive builder's don't think merit a change in model designation. "Railroad Model Craftsman" published a very good two-part article on the U25B (by Bob Kenderdine and "Win Cuisinier", with HO-scale drawings by George Losse.
They defined five "phases":
Phase I: very early units with ladders instead of corner steps (most of these are high-short-hood, but they included GE's first low-nose demonstrator in Phase I)
Phase IIa ("Classic"): units built between 4/62 and about 4/64, with one-piece windshield, handrail stanchions mounted on top of walkway
Phase IIb ("Late classic"): built from about 4/64 to the end of 1964, differing from IIa in details of the arrangement of hood doors-- New Haven's first order (2500-2509) were built in October 1964 and were of this phase.
Phase III ("Transitional"): built in first four months of 1965, with two-piece windshield and handrail stanchions bolted to the sides of the frames (the latter a feature of later GE locomotives up to the present!), but keeping the level nose of the earlier U25B
Phase IV: built 5/65 to 2/66, differing from phase III in having the sloped nose (and externally like the early U28B of, e.g., the P&LE). New Haven's second order (2510-2525), built in October-November 1965, were of this phase.
(ii) As one would expect, comparisons of the U25B and the C425 have been discussed on the "Alco" and "GE" forums. Someone who sounded well-informed, some months ago, opined that the C425 might have been intrinsically the better locomotive (GE was still having reliability problems with the FDL engine, and they had eliminated from their control system some useful feature that was included in the control electronics they sold Alco!), but that GE's after-market support was so much better than Alco's that the New Haven gave their whole 1965 order to GE.
(iii) A railfan I spoke to in the 1970s who had been given a cab-ride in a U25B extolled their smooth riding qualities! Mind you, I think the locomotives he was able to compare it to were the SW-1200 and RS3M used on the Canal Line local freight....