If those are the only two options I'd go 35/400. If you plan on keeping power low for tow and want spool go with an S471. If you're willing scarafice a little spool for more top end flow I'd go with 35/40 over an S475. If money is and issue 3b's can usually be found for cheap. Make sure you know what ex. housing is on the turbo you're buying. The most common for the S400 is 1.32, but the 1.10 is nice for towing. There are some bigger ones out there that you wont want to use. The 3b's have multiple ex housings. The most common are between a 22cm and 27cm. Some are divided some undivided. There are bigger ones out there but IMO 27 is the limit for a DD. Sorry for the rambling, but hope it helps.