I am looking to lighten my pack. I was trying to decide on going with a zpacks arc haul or something like the gregory paragon. my main concern is while the zpacks is lighter, how much comfort will I sacrifice for the light weight?
Have you lightened your gear? In general lightweight packs do not carry weight as well. Most claim to max out at about 30lb and work best with a 15lb base weight (pack + gear) - (food +water + fuel) or less. The best approach is to get your base weight down first and then look for a lightweight pack. With a reduced base weight you may find your current pack is fine and you don't need really need to change it out.
More important than the weight of the actual pack itself is how well it fits you and carries the weight of gear you require it to.
I have not tried and do not own either of the packs you mention but have looked at the arc haul some. It is very light and quite adjustable and people say they really like it but for long trail use (eg PCT) there are some complaints that it wears out more quickly and for some causes bruising on the lower back. That may not be a problem for you, particularly if you are a more occasional and short trip user.
Packs are a bit like clothing and it's all about the fit. Given REI stores are now closed it's a bit pointless recommending going there and trying some packs with the weight you intend to carry but it is best if you can try a few on and get a better idea of what works for you.
I checked around and the Gregory Paragon 58 seems nicely featured and well reviewed and probably will carry more weight more comfortably than the arc haul.
Another pack in the same class that may be worth a look is the REI Flash 55.
The Arc Haul is "waterproof" because of the DCF fabric used although I still use a compactor bag as a pack liner and modular in that you can add the external pockets you want.
I am not an REI employee.
Both options are great packs, and @OldGuyot added in the Flash, also a solid option in this category. He also brought up the key point when it comes to comfort in an ultralight pack and that is base weight. I don't own the Arc Haul, but a regular customer of mine (and now triple crowner! woohoo!) does and I fitted him for it in store. He found that he had to reduce his base weight to sub-10lbs so that he would be comfortable in his pack with water, food, and fuel. It brought his total weight to around 15lbs.
The Paragon and Flash will have more padding and will be able to carry a heavier load more comfortably. But you will absolute sacrifice on the weight of the pack itself. I have the Flash 45 for women and I found that I can even carry my winter backpacking loads comfortably, which can get up to a 35lb pack weight depending on how long we are going out for. My pack weight in the summers is about 20lbs, base weight is around 15lbs. I personally found that I like my creature comforts too much to sacrifice for getting to that ultralight base weight, which is what ended me in a Flash instead.
This is only a sort of broadly used term, but in my head the Arc Haul versus Paragon is the difference between ultralight backpacking and superlight backpacking. Superlight being lighter than traditional backpacking styles, but not as drastic in its weight cuts as ultralight. (Again, not broadly used term - yet, I'm gonna make this a thing! - but you will see the differentiation in certain tents in particular.) Mostly this will come down to a decision around how low are you willing to go in your pack weight.
Hope this helps!
Reducing the weight of the pack contents is far more important in reducing the weight you carry than getting a lighter pack. What is important about the pack is fit and comfort,not its weight.