Alright then.
I think that the inherent question speaks more to a lack of experience and roster depth than anything else, which is an odd thing for me to hear myself say because I never approached Empires and Puzzles with the intent of getting to where I am now. But I digress.
Once upon a time, I too was at a stage where I’d approach every fight with a singular rainbow strike team that needed to be versatile and adaptable which is a laudable goal but kind of misses the forest for the trees. For better and for worse, Empires and Puzzles is a game that rewards deeper benches with a range of heroes that can fulfill different roles based on each particular need. You don’t bring the army to a naval fight; you don’t bring Seal Team 6 to a tank fight; you don’t send the CIA to shoot down an alien invasion fleet. To wit, and to toot my own horn a bit, I refer you to this post I’d done earlier today regarding the Masquerade Ball quest I’d been in:
The short version is that this is my mainline rainbow team built to prioritize durability over firepower:
And it struggled to finish stage 5 of 6 in a timely manner. It was a long, drawn out fight that I’d won but taken quite a while to do so.
By comparison, this was the team I’d crafted specifically to approach Stage 6 of the quest knowing the bosses and what they were capable of:
And it dismantled the bosses in under 2 minutes.
So ultimately it kind of depends on the situation, and really you’re neither “gaining” nor “losing” either Boril or Sonya. You have both and can use both depending on what the situation calls for. Boril excels in fights where the enemy raid defense and/or bosses are high on firepower but low on support and/or dispels. Sonya is more effective in fights where Boril’s passive “hit me!” approach doesn’t work. As an example, a defense team composed of Grimm-LittleJohn-Gormek-LiXiu-Tiburtus will get creamed by Boril, but a defense team composed of Sonya-Boldtusk-Cyprian-cMelendor-WuKong is going to give Boril a hard time.
So you have Sonya for one fight and Boril for the other.
Ultimately though they both have their roles, but how useful they will be depends on your development as a player and your options as well. Sonya will be more effective long-term since she’s the only non-Frida blue hero that dispels which becomes incredibly valuable when you start color-stacking offensive teams, and if you get her costume, that effect is compounded since she’s one of the few heroes that cleanses at all, let alone at fast speed or in blue. Boril is more effective in the short term and remains one of the best tanks in the game due to stats alone, but the longer you play, the more likely it is that you’ll outgrow him.
When I first started this game, my first 4* hero, my first 4* hero I ascended, and the key hero I depended on the most heavily was Cyprian who has the same skillset as Boril. Now Cyprian sits on the bench unused except for maybe 5th or 6th war flags if at all. Cyprian/Boril are more useful when you’re facing a huge power differential as you can turn that power differential against the defender/bosses, but as the power differential between players/stages starts to equalize, you’ll most likely prefer a more proactive approach. Sonya will serve you for longer, but Boril will have his day in the sun as he likely does now.
Unless you’re talking raid defense. There are few tanks that are better than Boril and practically none at the 4* tier. Boril will likely have a place as your raid defense tank for a very long time and may eventually be upstaged by someone truly exceptional like Richard or Ares or Guinevere or Ursena, but until that day comes, Boril has a home somewhere at least.
And that’s my two 
s.