The best answer to this is to leave the light as a light and have a qualified electrician run a three wire circuit (hot, neutral, ground) direct to your soon-to-be office. Your modern equipment needs it and disturbing any old turn of the century wiring is asking for disaster.
Having lived in an old house with Range, Pump and four circuits, adding any extra load to an ancient system without upgrading the wiring is a good indicator for having several fire detectors located in strategic places and keeping their batteries up to date.
Stranded
Flexibility
Stranded wire is more flexible than solid core wire, which makes it well suited in situations where there will be movement of the wire. It also makes routing the wire through a building a bit easier.
Higher resistance to metal fatigue
Where solid core wire will weaken and break, stranded wire holds up to being bent repeatedly.
Solid
Cheaper to manufacture
While this savings may or may not be passed on to the consumer, solid core wire is cheaper to make.
More rugged
Solid wire resists cutting and stretching damage better than stranded wire.
Lower resistance
Given a solid and stranded wire of the same diameter, the solid wire will have a lower resistance. This is because there will always be gaps between the strands of a stranded wire. A stranded wire of the same gauge, will have a larger diameter than a solid wire to compensate for this. When you buy wire, you'll always buy it based on gauge. Both solid and stranded wires will have the same resistance, the stranded will simply be a bit larger in diameter.
Easier to terminate
If you're not careful when stripping stranded wire, you can end up cutting off strands (usually only a problem with smaller gauge wire, or wire with lots of strands). Depending on the number of removed strands, the ampacity of the wire can be negatively affected. Though nicking a solid core wire while stripping can make the wire more susceptible to breakage, so this isn't a clear cut advantage.
Solid core wire can be used in push in ("backstab") terminals, more often than stranded. This can be a pro or a con, depending on who you talk to.
Stranded wire can be more difficult to terminate on screw terminals, as you can sometimes have escapee strands which don't end up under the screw. This again can affect the ampacity of the wire.
Because stranded wire has a larger diameter, it tends to fill up twist-on wire connectors faster (less wires allowed in the connector).
Unless there's a lot of movement within the home (RV, house boat, frequent earthquakes), or you're pulling the wire through conduit. I'd say there isn't much of a difference either way.
Best Answer
This is not an answer, but it's too long for a comment:
Consider the cost. A 4x8 sheet of Sheetrock ~$11. A contractor who has an electrical license is $75-150 per hour in my area. If you can find one who needs work. Some use small jobs as fillers but I find that a small job of updating a room can turn into a can of worms and take all day so it will usually be quoted T & M (Time and materials).
I find it easier to replace a panel than update old wiring, and the panel pays better, no crawling, + usually decent work area. (I don’t go under most houses - that is for the apprentices.)
Just trying to give you an idea of what to expect. Electricians are in short supply in the Pacific Northwest, and if the job is a PITA you might have trouble getting it done. Without opening the walls the job may cost 5×, but open the walls and the PITA factor drops and the price also along with the unknown factor.
You may find a hungry electrician out there but use caution & have a contract! Understand sometimes the cheapest quote up front may be the most expensive in the long run. Try to find someone with references, not just online stars.
And since this is a DIY site it may be a chance for you to learn and save a few bucks. Going from a 15 to 20 amp circuit is quite simple: you are just installing #12 wiring. Take photos of each connection prior to taking it apart. Mark wires with numbers if you think you will get things mixed up. After upgrading the wire size on the circuit snap in a 20 amp breaker and you have done it.
(In my state repair of a circuit - in this case just replacing individual components - doesn’t require a permit and can be done by a homeowner. Check your local regulations as from what I have read some states require an electrician for simple “device” outlet or switch changes).