Lentivirus, shRNA plasmid, or siRNA?
Choosing between Lentivirus, shRNA transfer vector, or siRNA, depends on what you are seeking to accomplish. What advantage is there for you to create a stable knockdown versus a transient knockdown? If you are running your RNAi on an easy to transfect cell line, under a transient RNAi event, then siRNA is a well defined, proven and effective method. If you are working with a primary cell, neuron, or generally hard to transfect cell, then lentiviral particles for introducing shRNA is attractive. The shRNA transfer vector alone in theory should be easy to work with insofar as culturing the cell, DNA transfection, antibiotic selection, and then collect data. However the true purpose of the lentivirus transfer vector is for packaging into lenti particles.
Establishing a stable knockdown phenotype is feasible with the use of lentiviral particles. Viral particles that have a VSV-G coat protein (Santa Cruz Biotechnology Inc.) will have broad tropism. Otherwise if you are studying effects that can be measured with transient knockdown, then siRNA is a more simple and straightforward approach. siRNA is a user friendly technique since you can calculate empirical moles of duplex/# cells or total volume (molarity), with a minimal number of steps and peripheral controls toward achieving results.
shRNA transfer vectors are ~7 kb DNA constructs that can give less stability problem since it is DNA instead of RNA. However there are considerable nuances in the actual design and cloning of these vectors, in the actual establishment of antibiotic resistance toward generating a stable cell, and in controlling for the specific silencing/reproducible results. shRNA has limitations due to the nature of having such a large delivery vector producing a small hairpin substrate, and over cell passages under antibiotic selective pressure (ie puro resistance may not = Pol III shRNA cassette expression).