GNU/Linux

cmp Subtitle1.srt Subtitle2.srt
diff -q Subtitle1.srt Subtitle2.srt

Windows

Windows has two text comparison commands, comp and fc. However, both work in read-only mode. You could open the files in Windows. Jump back and forth between DOS and Windows to make changes.

comp compares any two files and shows where any differences are. To use it, type the following at the command prompt:

comp file1 file2 /a /L

In the command, replace file1 and file2 with the names of your files. You'll need to type the full path for each file. comp will compare the files letter by letter. The results will indicate each difference, along with the line number.

fc also compares two files, but it can put the results in a more readable format. To use it, type the following at the command prompt:

fc file1 file2 /a /L

Again, replace file1 and file2 with the names of your files. And include the full path for each file. fc will compare the files line by line. The results will give you the first and last lines of each section that's different.