I will be traveling in England in September and plan to purchase a BritRail Pass. I cannot tell if there is a difference in purchasing through Rick Steves website or through the Rail Europe website. Thoughts??
Are you sure that you want/need a pass? Despite what it says elsewhere on this site, they're almost never a good deal compared to the cost of individual tickets purchased in advance. If you've already done your homework and decided the pass is right for you, then carry on, but if not, have a look at http://nationalrail.co.uk to price out the trips you plan to take and add them up. Ignore the hand-drawn maps that claim that it costs $140 to go from London to York, and so on. There's no need to pay anywhere near that much.
rockycat, the RS website links actually take you to RailEurope. RailEurope is one of the two authorized dealers for BritRail serving North America, so it has to come from them anyway.
I considered a pass for recent trip, and decided against it, as the advance purchase discount fares came up considerably less than a pass. For example we got tickets for London - York for £12 each, purchased 7 weeks in advance. If you have a fixed itinerary, it takes a bit of work, but you can purchase each segment easily through National Rail site, and get reserved seats while you're at it.
Just to add, when using www.nationalrail.co.uk to price individual journeys, check the prices of trains that are about 10-12 weeks away from the day you check. That way you'll see the cheap Advance tickets.
If you do decide to book individual tickets, make a diary note to go back to the site 10-12 weeks ahead of your actual journeys.
However the cheap Advance (capital A) tickets have to be used on the specified train.
Besides answering the question that the OP actually asked, folks have volunteered their opinions on whether the OP should buy the pass at all. So, I'll add mine, which is that I have no opinion as to whether the OP should buy the pass.
But I have some experience with the pass and have drawn some conclusions from that experience. There's value in the flexibility of the Britail pass, and how much value is an individual decision. I've used those passes a couple of times.
Several years ago, I got the London area Britrail pass, and I believe I might have been okay without it, just buying walkup tickets for my day trip excursions. But the convenience of that pass was more than welcome - I was on a much-needed vacation at a stressful time, and it was so lovely not to even have to think about messing with buying train tickets, just hop on the train and go.
In the fall of 2015 I did a two-week trip around the UK by train with a flexipass Britail pass. Covered a lot of territory: Glasgow, Edinburgh, York, Penzance, London, Brighton. And some day trips while I stayed in York. I didn't run the numbers in detail, but I know I got more than sufficient value for my money with that pass. I made last-minute itinerary changes that cost me nothing as to the train trips, including one change after I was on the ground in the UK (invitation to stay at the home of friends). Those changes would have cost a few hundred pounds in forfeited advance tickets and walkup ticket costs, but as I said, with the Britrail pass they cost nothing. In at least one case I bought a cheap walkup fare (Glasgow to Edinbugh) instead of using one day of my Britrail pass, because that route is busy and a short ride and quite affordable, aka cheap.
My 2015 trip was, I think, the ideal one for a Britrail pass. It was planned as a "ride the rails" adventure and I loved the freedom to choose a different departure time or even a different day. For someone with a more structured itinerary, who isn't going to change the train trips no matter what, or miss the train, advance fare tickets would be perfect. Especially if it's just for one or two trips.
Last night I was at the National Rail website pricing train trips for the current draft of my next UK trip - still in planning stages, nothing booked yet. It involves three longish - 2+ hours - trips. The Britrail (3 day flexi, senior 1st class) pass at £180 priced out competitively with the total 1st class advance fares for those trips, which was £157. Especially when you remember that the Britrail pass has complete flexibility vs. advance fares which commit you to a specific departure or you lose what you paid for the ticket. By the way, as best I could determine, the total walkup (purchased on the same day) fares for those trips came to £431. That's the cost of flexibility without the Britrail pass.
Indeed - for a holiday that involves intensive use of long distance rail travel, particularly first class at peak time the BritRail Pass can a veritable bargain. For those doing three or four point to pointers it probably isn't. Without any knowledge of an itinerary it is impossible to advise on this.