Neither. For about the same grades, you can go to Leicester University which, according to Edurank, is the 26th best Law school in the UK (46th in Europe). They say that Surrey (which became a 'proper' university in 1966) is the 35th best in the UK (68th in Europe) and that Royal Holloway is the 47th best in the UK (119th in Europe).
If you prefer Reading University for about the same grades, that's 29th best in the UK (53rd in Europe).
As universities overall for all subjects on average, Edurank ranks Leicester 24th best in the UK (97th in Europe).
They rank Reading 28th best in the UK (109th in Europe).
They rank Surrey 31st best in the UK (123rd in Europe).
They rank Royal Holloway 44th best in the UK (231st in Europe).
According to The Complete University Guide, Reading has the highest research quality for Law of these (73%, joint 40th in the UK),
Leicester and Surrey both 72% (joint 48th),
and Royal Holloway's was 'not available'.
Leicester Law School takes about 515 new students per year. I couldn't find figures for those other universities but Leicester is one of the largest Law schools in England. For comparison with some others (new Law students per year):
Liverpool 590
Nottingham Trent 555
The University of Law 510
Northumbria 505
Leeds Beckett 500
Bristol 400
Exeter 400
Sheffield - as they have 1100 Law students in total, no more than 370 a year on average.
King's College London - As they have 1048 Law students in total, no more than 350 a year on average.
UCL - as they have 825 Law students in total, no more than 275 a year on average.