Totally on the same page. I'm currently looking at
https://www.perennialcycle.com/shutter-precision-upgrade-dynamo-package-for-brompton.html. The SON package costs another $280 which I can't justify as I'm never going to do all-night rides on my Brommie.
Oh wow! At the price for the SON at perennenialcycle seems pretty high at $480! I do only know the German prices and over here a Brompton SON wheel is 260€ (w/o lights) while the Brompton SP8 wheel is 175€ (bought solo, not together with a bike, it might be 25€ less, not totally sure currently) and the SP9, built into a wheel is 200€ish. So a recognizable but not overly massive difference in price. This difference becomes pretty irrelevant if you take into account how long you'll use the bike and the dynamo hub. Even if you calculate just 10 years of use the difference shrinks to 7,50€ per year - plus you can be sure that the SON will massively outlast this period, no matter how much or intensively you'll ride while with the SP it might or might not be. Also you get a 5 year warranty on the hub. So to me the SON is simply the better investment (if you can handle the initial price tag to buy it).
With the SP you can buy the SP8 and the SP9 solo w/o a wheel (and have to with the SP9 in Brompton size) which costs 80€-120€ for the SP8 and about 90-130€ for the SP9, depending from the market situation and from where you buy. You will have to build a wheel around it then, so you'll not end massively cheaper than buying a complete wheel if you don't built it yourself, given the price for spokes and rim plus the labor. And if you look closely you'll recognize that the extra that the SON makers charge to have the dynamo built into a wheel is just 50€ difference to the naked hub (209€ vs. 259€). A bit of a bargain, especially given the fact that those wheels are professionally hand built in the SON factory.
The lightening options are obviously identical with all dynamo hubs and influence the final price massively, depending from your choice. While a standard backlight is perfectly ok and around just 20€ (if you decide not to go for the shiny SON one for 60€) the front lights do have a broad range, with better BUMM lights starting around 40€ (EyeC) decent ones like the BUMM X or XS being 70-90€ and the SON Edelux being around 140€. Easy to start cheap here and to upgrade later - this could solve your financial issue.
Putting everything together the complete SON package with hub, front- and rear light adds to ~450€, with a standard rear light you end up a tad over 400€ and with a standard front light you might end up around 350€. With a SP hub the same packages are about 50-70€ cheaper - which for me is not enough of a difference to choose the SP instead of the SON hub.
Now the choice is up to you!
