If you have back problems already, I would not recommend performing these two exercises. Stick with the machines that target the hamstrings. Machines to do not offer the same intensity as the two exercises you mention here. However, you can increase that intensity by throwing in some supersets, drops sets, slew reps etc.
If you do not have a back problem, you can and should implement these exercises into your routines. SLDL are the ultimate exercise for hamstring development! However, form is everything.... and if you do not know proper technique, these are risky exercises that can lead to injury.
If you are going to try SLDL (stiff leg dead lifts), and have never done them before, def. start out with a light weight so you can master the form of the exercise.
Grab the bar with an overhand grip and keep your feet shoulder width apart. (some like to go a little closer). Bend at your waist and DO NOT arch your back. Keep your knees nearly locked and look straight ahead. If you are flexible, you will be able to lower the weight to the floor. If you are able to do this, you should stand on an elevated surface such as a box or bench, because you do not want the path of the weights to touch the floor.
However, this should not be your goal. You should lower the weights, while maintaining a straight spine, until you feel the hamstrings being stretched to the max. This will be different for everyone. At the most, lower your torso so it is parallel to the floor.
It is even better to find a trainer or advanced lifter in your gym that can show your proper form. But this is a start.
If you have any questions, let me know.
P.S. If you do have a back problem, I would also throw some lunges into your leg routine as they hit the hams as well.