I have trading and ISA accounts with both HL and TDW.
The sites are comparable in cost and service for UK stocks, albeit TDW are slightly cheaper but HL are better for Unit Trusts, (better choice and cash back on trail commission).
TDW are much better set up for international trading with a cheaper service and in contrast to HL they will do the admin to facilitate reductions in withholding tax. This HL refuse to do, costing money to the investor especially in ISAs.
So far as their individual trading sites are concerned HL is easy to use and have never made a mistake for me, but international has to be vis the telephone. TDW's site does suffer from the occasional glitch which is resolved by phone but is open for international trading.
You pays your money......