Depends on the type of shelter you are trying to put together. Straight up bivy alone or under a tarp of some sort?
I will be using the Bristlecone Bivy (8oz & about $150 new) under a Megatarp. Not a waterproof bivy since I will rely on the tarp for that. My main reason to use this bivy is to protect my sleeping bag from debris and ground moisture.
I take my MSR Hubba when I'm expecting rain everyday, which is not very often. Now I'm making the switch from the North Face Aslym Bivy tent to the Outdoor Research Advanced Bivy. Its a bit tricky getting in and out of the North Face Bivy tent and I'm not sure if it would handle harsh weather all that well. I'm leaning toward using the OR Advanced Bivy no matter the weather with a small 5X8' tarp. Tarps cover gear as well as anything else and I have found a small tarp is useful when glassing in inclement weather.
Definitely check out REI. Try out tents, bivy-tents, and bivy's to figure out which one best suits your desires and expectations. If your not satisfied with the tent, then simply return it for bivy or bivy-tent. Buying every kind to have can become expensive very quickly.
I have an old OR bivy I got for climbing 14ers and it is still bombproof.
However, if the forecast looks bad at all, I prefer having a roof like the ParaTipi. It's uncomfortable getting trapped in your bivy for 14 hours because a storm front comes in and your only warm option is inside the sleeping bag.
OR Aurora Bivy with a home made tarp. There are much lighter bivy sacks out there now but the OR has been good to me. It weighs 1.5 pounds and costs $200 ish. It is full Gore tex and has proven itself in a few beat downs. Not sure the weight of the tarp but it is made out of almost the same material as the Kifaru tarps/tipis. If I were going to buy a tarp tomorrow it would be a Kifaru for sure.
Lawnbio is correct the top of the TI isn't waterproof, but the bottom is. Being as you are looking to use it in a paratipi the non-waterproof is a non-issue. In fact a non-waterproof top of a bivy is preferred for me to help it breath and not build condensation.
What functionality are you looking for in your bivy, keep your bag clean, protect from rain, increase the warmth of your sleeping bag? The reason I ask is, and this is really directed at everyone, since you will have a tarp as your main weather shelter, would a sleeve of tyvek that you put your sleeping bag and pad into serve the same purpose as these other mini-tent bivy sacks .
My main issue is with water coming in under tipi during heavy rain and condensation not sure of the chances of that but like the peace of mind so I guess you could say I need it to protect the bottom of my bag and be lite
Correct pitch location is key, as with any tent. Iv your in a not soaking wet climate, and pitch your tipi in the right spot, you will not get. If you pitch it in a depression you will get wet as with just about every other tent, floored or not. Pitch it in a good spot and forget the bivy is how id go. Only reason id like a bivy is to increase temp and protect my bag in floorless shelters. Water entering has not been a concern.