To get the best scaling prioritize like this:
1. Use integer scaling or something with "single digit" ratios (i.e. 2:1, 3:2, 4:3 etc).
2. Use as many lines as possible (i.e. 512 line look better than 480 lines)
3. Apply a scaling effect.
If you apply this to a 384 line 25KHz game, you should use a resolution with 576 (3:2 scaling), 512 (4:3) or 480 (5:4).
Note that your 15KHz horizontal games will get borders with 576 and 512 lines so you have to check if the improved display for 25KHz games (and vertical 15KHz games) is worth it. I do.