hey Joe
ive had experiance with both markers and for me, i go with the Delta only because for me its lighter, can add a rip clip to it (dont get on with cyclones very well
) and doesnt seem as air hungry as the Sierra. but thats just me
if you can, try them both either in a shop or a walkon site. get a feel for each marker
each marker has pros and cons
Sierra
a tad cumbersome due to length (shroud can be removed and stock is folding so easily solved
), stock barrel for me was a little iffy (inaccurate at mid-long range and sometimes close) but can easily be replaced (98 thread), love the folding stock.
recommended upgrades: cyclone or rip clip (rip would need to be the 98 version to fit proper as im led to believe, need to confirm), stock barrel to maybe a apex or even a Hammerhead and maybe remote line the air tank (would need tank pack or tac vest). i think you can possibly get a reg or maybe an on/off but unsure
Delta
essentially a Combat with Delta on name plate and MP5 shroud and mag. stock barrel a little short for my liking and has been inaccurate from time to time (so i changed mine xD), nice sliding stock and like the Sierra, good milsim look
recommended upgrades: RIP CLIP
almost a must have for any BT marker! (unless you have an elec hopper e.g. rotor, halo etc) good solid feedrate and easily stops no feeding or chops in the chamber (chops, it can happen but very rarely for me ),stock barrel for maybe apex, hammerhead or even a freak kit (BT/A5/X7 thread),