Here are links to the properties you are interested in (there are google maps with actual live street views):
In regards to Myrtle Street, these are brand new townhomes that seem nicely upgraded and staged. Unfortunately because they are so new there is no sale history for other units (no direct comparables). When I did a quick search of townhouses with the same square footage I found 10 that have sold in the last year but none over $390,000. I took a fast peak at what these looked like on the interiors and according to the pic's none were as upgraded and new as Myrtle so that is likely where the extra $29,000+ is coming into play. I think the property is nicely priced as you get the garage parking, new amenities and an extra deeded spot. Generally speaking, Watertown has a pretty healthy townhouse market due to its proximity to the city. In terms of future value, I wish i had a crystal ball for you, but Watertown tends to have a pretty healthy market, again due to its proximity to cambridge (harvard and MIT) and Boston (schools, economy) and those things aren't going anywhere.
8 School St is also new construction and seems nicely done (amenities), close to the T. This property has been on the market for 288 days and has not had a price drop. The price per square foot is vastly different (better) in comparison to the Watertown property - twice the space. It is a little further out (from the city) than watertown and therefore the townhouse market might be slightly slower than the watertown as it is more suburban (families). I think Waltham is an interesting investment ... There have been a lot of local renovations and influx of corporate offices (tech) which is good for the local economy. I checked sold data: townhouses, last year, similar square footage and there were 11 sales, including a couple on school street. The highest price in this square footage range was $515,000. the other townhouses in the complex with similar square footage sold at $218 a square foot and one of them sat on the market for 300+ days. Who knows why 8 school hasn't sold perhaps it is as simple as a cool down in the market and the days on market being higher for this area - things simply take longer to sell.
I hope this helps!