15

Between Bucuresti and Sofia.... Mmmm.
There'll be a few differences of opinion here. If you've 5 days between the two cities, then go in between ! !
I really like Ruse, more so than Veliko Tarnovo (a lot of peoples favourite Bulgarian city).
As already suggested, with an early start, Plovdiv is a good option too. It's only about 120kms from Sofia.
There's a train at 06:55, gets into Plovdiv at just before 10:00.
That is on the existing timetable though, which changes before your date.
Having to choose however, between how many nights to spend in both Bucuresti and Sofia. I dunno.
I've done both capitals many times. Neither of really float my boat much.
If I really 'Had' to choose between the two, probably Sofia. There are, for me, nicer day trips out.

16

I might still stop in Ruse for a day but I will sleep in Sofia as I have booked the Sheraton Sofia Balkan hotel for free (using my points)
so 3 nights in Sofia it will be then.

17

Good choice.... Good trip ! !