Heathrow express

Currently 30 days minimum for an Advance ticket.

... but what even the Heathrow Express website doesn't tell you is that between 14 and 30 days in advance there is a 2nd tier of discount at £22.
So only within 0 to 14 days do you pay the full fare.

Maybe worth mentioning, while many immediately think "Heathrow Express", it really is only useful if you are going to Paddington, or staying near there. The Elizabeth line (which also stops at Paddington) goes further into the city, and of course the Piccadilly Line is much better for Hammersmith, Baron's Court, Kensington, Victoria Station area and the South Bank.

It is not a legal requirement to enter London from LHR at Paddington Station so you may have better options depending on your final destination.

Nor is it a legal requirement to enter the UK at LHR, so many don't understand that! But we'll accept that Katrina does need to be in London.

The problem with Heathrow Express is that it is not as clever as it seems. For a start, if you arrive at T4 you have to take a free ride on an Elizabeth Line train to Heathrow Central then get off, it carries on whilst you wait 11 minutes for HX to collect you and chase after it. If all goes to plan you arrive 1 minute later than the train you were originally on!! And if you didn't book early enough for the £10 ticket you'll pay more for the inconvenience.

If you get down to the platform at Central and just miss an HX to Paddington you will have a 15 minute wait for the next, but Elizabeth Line will arrive in 4 minutes and get you to Paddington 1 minute earlier than the train you stood waiting for. This is especially annoying if you didn't book in advance and just missed the HX because you were buying the more expensive ticket - if you haven't booked in advance use "pay as you go" and just jump on whatever comes first (or don't use HX).

The T4 issue mentioned above is particularly annoying, and something to consider for the return as it adds extra time. Last year I almost missed a flight, despite arriving at the airport much earlier than usual, because the Heathrow Express got me to T2/3 but there was an issue with the connecting tunnel to T4 and all those trains were suspended. The only other way to get to T4 is one of the local buses but they're not very frequent and were so crowded I had to wait for at least three to go by before I could board one. Of course, this is only a concern if you're flying in/out of T4.

Here’s my rationale for the Express or Elizabeth line. I fly from LA. Overnight flight and the LAST thing I want to do is dawdle… that and for convenience regarding jet lag I book 63 Bayswater. For decades was the cheapest place to stay ( no longer the case) but it’s a convenient stroll from Paddington. I know the neighborhood. Great pub grub at the Mitre or the Victoria. Across the road from the Lancaster Gate into Hyde Park. As an early riser I love that the park gates open at 5am.
Lots of places to get coffee whilst I stroll around the park and in the neighborhood. I won’t EVER have a Starbucks so knowing I can stroll to the garden cafe by the Serpentine and amble a bit further to a Blank coffee is ideal. FYI I also bring my own ground beans and 1 cup travel coffee filter and cones. I do this because there is nearly always a self heating kettle in accommodations
( something one should research when booking a room ) So if I return to London and 63 I’ll purchase HEX tix 6 months out. 3 at the latest.

Now if I don’t stay at 63 I’ll investigate an apartment. Last trip enjoyed my stay at the Club Quarters Hotel at Lincoln Inn Fields. Apartment was on the 5th floor. Very quiet, simple view of Lincoln Inn Fields and skyline. Room had a setee which was a great spot to sit and catch up on emails as well as uploading iPhone pics to my Instagram account. Have already researched their other property locations. I did like the location as it was 7 minutes to Holborn Tube Station. Also loved the pasta meals I could get at the Seven Star Pub. Not to mention making friends with The General.

If I return in November might stay there or try one of there other properties. Also considering another Premiere Inn in an area I’ve never explored. Like Muswell Hill or Pimlico.
We shall see.

I use Heathrow Express because it has plenty of room for luggage, and I purchase my ticket in advance to get the discount.
It works for me.

As has been said above, the HEX is fine if you are buying a discounted ticket and your final destination is at or very close to Paddington. If your destination means taking a tube, bus or taxi from Paddington to your accommodation any advantage with HEX rapidly disappears both on cost and convenience grounds.

Not the original poster, but thx much for recos for those staying somewhere not near Paddington. Very helpful!