The short version
Bang Si Mueang is a low rise river side area of Nonthaburi, quieter than the Rattanathibet corridor and more dependent on local roads. It suits residents who want an established Thai neighbourhood and do not need to be in the centre every day.
The most appealing parts are near the river, temples and older communities. The practical parts are closer to the Purple Line and main roads. Those qualities rarely sit in the same doorway, so view both the home and the route to it. A riverside address may be calm but inconvenient for groceries, school or a late return.
The Purple Line at Bang Rak Noi Tha It is useful, though the area between station and home may require a motorbike taxi. Cars make family life easier and river crossings less frustrating, but congestion is part of the trade. I would choose Bang Si Mueang for its local pace, not because it is a secret shortcut into Bangkok.
Verdict
This is a good fit for a household with a settled routine and a preference for older communities over towers. It is less convincing for a newcomer who wants rail, nightlife and English language services immediately outside the building.