I have never used either. I need an LHR hotel next Sept. How do I know if hotel I get is on Hoppa bus line?

It is far too early to be bidding PL for next Sept - so you have lots of time to study biddingfortravel.com and betterbidding.com between now and next Spring.

Most all LHR area hotels are on a Hoppa route. And even IF you were unlucky enough to get one that isn't, the short cab ride wouldn't cost much.

And lots of study is necessary before you bid.

We had a dayroom at Novotel, and took a taxi. Since there were 4 of us, it was economical and faster.

I would wait for some time before getting a hotel via PL or HW.

If you want a peace of mind, book a refundable LHR hotel for now.

So many things can happen between now and next SEpt (10 months away) - airlines can change their flight schedules, can cancel the flights etc etc etc.

The following website is one I find the most useful. It lists the LHR hotels and which Hoppa line serves which one. It also lists LHR hotels which is served by the FREE local bus.
http://www.londontoolkit.com/lhr/hotel_hoppa_bus.htm

I agree with all of the above! Too early and get a tentative reservation.