We had the same dilemma when our son was younger, T5 with pop top or Ducato/Crafter size with bunks.
In the end we opted for the T5. The key benefits for us were; reasonable size vehicle for everyday use or short trips away, but still with two double sized beds, low overall height for getting into car parks with height restrictions. Able to put son to bed in the roof and still use downstairs.
Downsides; not really suitable for all year round without some kind of heater due to canvas roof. But we used external insulation to get around this. Not much storage space inside for bikes etc with a crashworthy rock and roll bed fitted, so less secure.
We ended up going T5 LWB (40cm longer) to give us some extra space. Doesn’t sound a lot but made a big difference.
Son was 3 when we got it. He’s now 15. So has served us well. But he’s now too tall/long for the roof bed!!
If you want to go for two full sized beds without a pop top you used to have to go for a fairly big base vehicle. However, there are some clever solutions now with beds that lower from the ceiling which are worth looking at.