It’s really down to your preference.
1oz will mean, in theory lower premiums, but decrease liquidity.
Fractional will mean, in theory higher premiums, but increase liquidity.
Arguably though, if/when you come to resell, if you do so at a fair price point, they’ll likely sell either way, just might take a bit longer selling the 1oz.
What you could find, is that if you do decide to go down the fractional route, then the higher premiums you’ve paid, can be passed on when it comes to selling them on in a private sale, say to someone on TSF. Broadly speaking they’d understand that fractional carry a bit of a higher premium, but as long as they can see it’s still cheaper than a dealer.
Very broad brush advice I know, sorry.
Personally, I’m going down the fractional Brit route., just on the fence between 0.5oz or 0.25oz’s. Though I’ve seen how quickly sovereigns get snapped up on TSF, so if liquidity is something you value highly, then full sovereigns is a very good option. (When I say sovereigns, I mean proper sovereigns from the Royal Mint)