The Hilton Paddington is an expensive train trip to a pricier property with a poorer lounge than either of the two airport Hiltons. The OP is a Gold so likely to get lounge access.

Whilst HGI's can be good they are not really full service properties and either of the two airport Hiltons are better options than any other HGI I have visited.

The T5 Hilton is further away (best option Uber to get there and back) but has better rooms and public space. The T5 Hilton has a great Indian Restaurant

The T4 Hilton has very small rooms unless you are lucky enough to get a suite which are very nicely laid out. The T4 Hilton has a great Chinese Restaurant.

The T4 Lounge is I think bigger and better than the T5 lounge.

For me I think it depends on how sensitive you are to transport costs and if you have checked bags with you. If I had bags I would probably pick the T5 Hilton and uber both ways (but you could equally do this with the T4 property, it just doesn't have much space for bags in the rooms). If I had hand luggage only I would pick the T4 Hilton if it were at least £20 cheaper than the T5 Hilton and use the free buses between terminals.