Depends what you're comfortable with.
Old Sarum's a great place but then again so is Popham.
OS is likely to be pricier than doing it at Popham but then OS is licensed and has conventional approaches. Popham has interesting approaches and two runways rather than one.
Membership costs at both are similar but Popham's includes all landings while OS now includes landings only in club aircraft.
I'd suggest a visit to both and make up your own mind.
Oh and OS sits inside the Boscombe/Wallop CMATZ, which also incorporates the Porton Down restricted area so your options for entry/exit are more restricted.