Well there both really nice the P25 and the Plus. The Plus gives you some advantages over the non -plus . For all the backs the LCD is better on the plus It is brighter and has sharper reviews also. The files look great but Phase has worked on some things to improve the plus series. Here is what Phase says from Lance
Screen the non-plus is reflective LCD (better outdoors) and the plus backlight LCD @ about 2x the resolution approx 240,000 pixels.
-ISO non plus 50-400 (on P20,P21,P25,P45) and 100-800 (p30) vs 50-800 (on P20+,P21+,P25+,P45+) and 100-1600 (P30+)
-longer exposures on the plus series
-unlimited burst lengths tethered or untethered.
-Live preview standard on all Plus series, most refurbs come with it except P25's .
-Plus series has OptiColor+,Dynamic+, and Xpose+ which all mean improvements in image quality.
Now the biggies are the LCD and the ISO improvements . I get pretty clean ISO 800's and you will see a ton of them next week from the runway. The non Plus is good at ISO 400 and slower for the P25 and if you don't need the higher stop than a really good deal . For landscape and outdoor shooters the plus is not going to really buy you that much . Better LCD but you will still run for shade to see the LCD anyway. I used the P25 for about 3 days in San Juan along with the P30 plus and honestly you could not tell much of a difference. I think the P25 is a nice stepping stone into the MF world. i started with the ZD and although good I should have just went for it with Phase instead. Now the P25 does do a pretty darn clean ISO 400 and that maybe all someone truly needs. Now I just looked at the price of the P25 refrub. with new body and lens for 13k that is a steal. The body and lens on the street when Mamiya brings it out will be over 4k alone. BTW for the P21 plus new with body and lens there is a Phase promotion of 9900 euros which is more like 12k or something like that but the P21 is a crop sensor like the P30 Plus and 18 mpx. A lot of this depends on what you plan to shoot also.
To me I tried the P25, P30 plus and the P45 plus. I liked them all but I went for the universal performer and cost savings from the P45 and got the p25 plus and there is something about the 9 micron sensors i really like and that has to do with the artist side, just has a nice look to it. Now you have to decide between the P25 and p25 plus than you have to see if the 6 to 8 k difference is worth it to you from a refrub. to a new setup of the P25 plus because trying to find a used p25 plus is a little tough to do. I figured I would go the Plus because i actually plan to hold on to this for a little while. I know very strange words coming out of my mouth but the is not a world of difference between them but a big step up the ladder from 35mm.