I am building 2 semi-hollow arch top basses 1 @ 30.3"scale the other 34"scale.
This is a mechanics of hardware post, if not off-topic, focus: strings. Whatever scale length you choose, there will be players for both of your recommended scale lengths. The topic of this post is string length. Being that semi-hollow is the anticipated construction, what kind of a tail piece are you contemplating, how far back from the bridge saddles will you contemplate the string anchor, and how are you designing the headstock?
The reason for these questions is that I would suggest you contemplate the ball anchor to tuner side of nut length very carefully, so that you don't unnecessarily limit yourself to the makes, models and gauges of strings available.
For example:
A Harmony H-22 is a short scale bass, but the anchor is at the tail of the instrument, so this bass uses regular long scale strings.
A Hofner 500/1 is a 30-inch short scale bass. But because of the distance from the bridge saddle back to the ball anchor, either a La Bella special "Beatle Bass" set or what other manufacturers call a medium scale set are necessary.
A vintage-style Mustang is through-body. So again, a short scale set strung through body may be too short, and again, La Bella makes a 760-MUS set for the string through.
By contrast, a Gibson EB-3, top-load MusicMaster and Bronco, and others, take a "true" short scale set.
On the other end:
An Epiphone Jack Casady bass is a 34-inch scale, but because it uses the compact 3-point bridge, it uses regular long scale strings.
If the tailpiece ball anchor is separated from the bridge too far on a long scale bass, even extra-long or super-long scale strings may not have enough wrap to clear the nut.
Please consider these items, including consulting manufacturer's winding length charts for their various scales, when designing your bridge and tailpiece setup. Thanks.