Nemo Disco 15 vs 30

Hello, I am getting my first proper sleeping bag.
I’m trying to decide between the **NEMO Disco 15 and Disco 30** as my main 3-season sleeping bag and I’m starting to overthink it.
My use case:
Mostly **late spring, summer and early autumn**
Roughly **50/50 lowlands and mountains.**
Nights could range from around **0–5°C (32–41°F)**
**on colder mountain trips to 10–18°C (50–64°F )**
**in summer**

I am a mostly warm sleeper but i prefer to sleep with no additional layers.

**Would you keep the Disco 15 for the extra safety margin and vent it during summer, or swap it for the Disco 30 and layer up when temperatures get close to freezing?**
Especially interested in experiences from people who have actually used either Disco in **10–15°C / 50–60°F summer nights**. How effective are the Thermo Gills at making the 15 comfortable in warmer weather?