Source & Meter

Advantages compared to LTS:

More flexibility. One source & meter can measure a link, whereas two LTS (Loss Test Set) instruments are needed.
A source is not needed to do transmission power measurements, so it can be used elsewhere.
The KI7600 series power meter has a built in visible fault locator option.
A single source instrument can have both multimode and single mode emitters.
The source and meter should be matched so that "Autotest" works.

Disadvantages compared to LTS:

Higher initial cost. One LTS may be cheaper than a source / meter pair.
More inventory to maintain and deploy, so higher ongoing costs.
Higher battery cost.
Slightly harder to use.

Basic Loss Test Set

Advantages compared to source and meter:

Maybe lower initial cost.
Less inventory to maintain and deploy, so lower ongoing costs.
Lower battery cost.
Slightly simpler to use.

Disadvantages compared to source and meter:

Generally, one unit can't verify a link, two units are required.
Source (cost) must be carried during power measurements, when it's not required.

Two Way Instrument

Advantages compared to LTS:

Much easier & faster two-way (bi-directional) loss checking with minimum skill level.
Pass / Fail display.
Option of length or ORL measurement for small extra cost and no extra effort.
Can test all fibers in an installation to the highest level, so networks can be easily re-configured without having to re-test.

Disadvantages compared to LTS:

Initial Cost.

When considering these options, remember that the Total Cost of Ownership is usually dominated by labour, calibration, training, inventory management and other indirect costs.