"nced"
"Oh yeah, why not metal guides?" Because "plastic" guides dampen twang/vibration better than metal guides. For max velocity from a spring, a hard steel guide surface that's tight fitting to the spring and lubed with molly paste is more slippery than a tight Delrin guide with a softer surface in the same spring. The slippery hard surface of a metal guide also makes "twang/vibration reduction" more of a challenge since the spring coils continue oscillating on the metal guide after the shot.
Since I never try to get "max velocity" with my HW95/R9 tunes anyway I use a thinner wire spring giving a nice 12.5ish fpe power level (7.9 grain pellet at about 850fps) which works well with my HW95/R9 weight springers. Also, as I've mentioned in other replies.......in a couple decades I've never had a failure with my tight fitting Delrin guides..........
"And is the a preference in piston seals too?" Decades ago I started cutting my own aluminum oring sealed piston caps for my R9s because the combination of the factory thin edged HW parachute seal, molly paste, and spring tar created poi issues during a shooting session (like a field target match) if the shooting temp variation exceeded about 20F. When I switched to oring sealed pistons "way back when" I could pick up 40-50fps with the same spring vs the factory HW piston seal. At that time I switched from .128 wire springs to .120 wire springs because I could still maintain my favorite velocity with easier cocking, less gun motion, and less scope trashing. For me "form follows function" so in the beginning I would tune my .177 R9 to shoot 7.9 grain CPLs at around 910fps. The reason for "910fps" is because my scopes at that time only had duplex reticles and I learned that when my gun was zero'd at 30 yards the tip of the "lower reticle post" would be on the poi at both 10 yards and 50 yards. For the intermediate distances I would bracket the killzone between the cross hair and tip of lower post. For the 55 yard targets I would put the tip of the lower post on the top edge of the killzone and the pellet would go through the killzone. When I started using "dotted reticles" (mil dot and such) I had more aiming points to accommodate a loopier reticle so my normal velocity has been reduced to the 830 to 860fps level depending on the "urge of the moment". LOL....I think that I have as much fun messing with my springers as I do actually shooting them.
Anywhoo....while I still fit my springers with oring sealed piston caps I did find that the later HW piston seal design with thick parachute edge and reduced "waist" performs so close to my oring sealed caps that it really isn't necessary to go through the oring sealed cap route. Here is a pic showing HW piston seal design over the years...........
Here is an early oring sealed piston cap.........
Here is one of my newer piston caps where I've added a Delrin bearing ........
There is another benefit for oring sealed piston caps for my HW pistons.......the face of the cap doesn't erode at the transfer port like this.........
However even the erosion issue seems to be mitigated with the new design (new as of a few years ago) since this pic shows almost no "wear" at all........
I do have to mention however that the newer HW piston seals vary quite a bit in fit to the receivers with some fitting too loose (for my taste) and some fitting correctly (IMHO). When tuning a HW95 for a friend I found that the piston seal of his new gun was rather loose fitting so I took another new HW95 piston seal from my parts bin and it fit perfectly in the same receiver.
Since you mentioned piston seals, I found that the Vortek VAC seal worked well in my R9 without the need for sizing, however a Maccari Hornet seal was made from a harder material that needed to be "sized to fit" unless I wanted a 1000 shot break in period. Of the three molded seals, I personally prefer a good fitting HW factory seal if I weren't using my oring sealed piston caps.