Funnily enough I have been going through the same dilemma. I don't have a huge budget so these fractional coins suit me down to the ground. I have bought one of each literally to help me decide. They are both small coins but the detail is still amazing. The half sovereign is a little bit bigger but a bit more expensive. I have done a few sums and as long as you buy bullion grade you are paying about the same per gram so it really comes down to preference. You have the 200 year history of the sovereign which means alot of choice, alot of royals to pick from, 2 main reverse designs (shield and st George) but also some interesting one off coins 1989, 2002, 2005, 2012, 2017 etc. Obviously you then have all the rare coins too. The Britannia is fairly new 1987 but also with some interesting reverse designs and rare mintage years. I've decided to look at both and just get the best coin for my money at the time I have the money ready. Proofs, rare coins and graded coins will have to wait for now.
Good luck with your choice 👍