T line VS Reservoir?
 

What is better?
Does a reservoir increase water flow restriction in the loop? As i would assume it does - is this significant?
Your thoughts will be appreciated.

You don't "need" a reservoir, but the added cooling you get from more water volume would probably discount (or even outweigh?) the detrimental affect of having slightly lower flow. On top of that, you may find that it's more convenient to fill your system with a reservoir in the loop.

Of course the guys that test this kinda stuff with real measuring devices would be able to give you actual numbers.

>> What is better?
Both have there pros and cons.
Res's aid filling and make bubble removal a sinch. Installing the res just prior to the pump is best (almost the only "proper" location) as it allows you to run a non-restrictive inlet to the pump which can make flow rates improve slightly from a setup that has a restrictive pump inlet. A res located elsewhere in the loop (say immediately after the pump) is going to cause a very slight restriction (almost entirely from the barbs). This location is inherently dumb due to the presurisation of the res and probably leaking, but that's another kettle of fish. Lastly, reservoirs provide a great place to measure coolant temp.
Reservoirs take up room and may cause related plumbing complexity. Any other CONs anyone?
A T line is a very small reservoir. It doesn't slow the coolant at all at the branch, so it doesn't provide the same bubble catching ability, but still does catch bubbles. Filling is relatively simple depending on exact implementation - typically the same as having a res. A T like typically is not used to provide a low restriction run to the pump inlet, so you use that. Measuring temps is provided to a small degree. T lines immediately after the pump will be presurised. T lines are tiny and typically easy to plumb into the loop.
So, you get reduced benefits and skip one or two benefits, but its easy to fit and plumb

> Does a reservoir increase water flow restriction in the loop? As i would assume it does - is this significant?

As per above, it can. A good reservoir allows a low restriction pump inlet run and thus can aid flow rates. Shame those bay-res's don't have 16mm fittings...

> the added cooling you get from more water volume

You don't get any real improvement to cooling from having more water volume. It will take slightly longer to reach the stable operating temperature, but the volume of water will not greatly impact this temperature. The reason I use vague terms is naturally the more tubing you have and the reservoir itself all of which act as crap radiators (but radiating some heat none the less).

> Of course the guys that test this kinda stuff with real measuring devices would be able to give you actual numbers.

I'd love to do a test with a BA rad (120.2), a LRR (or similar high-restriction block) and a common pump (MCP600 or eheim 1250).
Test 1: 1/2" 90° fitting on pump inlet, no res or T -> flow rate (worst case)
Test 2: 1/2" straight fitting on pump inlet, no res or T -> flow rate (worst case)
Test 3: 1/2" straight fitting on pump inlet, T line on inlet side -> flow rate
Test 4: 1/2" fitting on pump inlet to bay reservoir using approx 30cm of tubing-> flow rate
Test 5: 16mm fitting on pump inlet to reservoir with 16mm barb using short run of tubing (ie 5cm or less) -> flow rate (best case)

I bet the change from worst (test 1) to best (test 5) is like 1 LPM or less.