Plaxis 3D Ultimate or LE?

PLAXIS uses Finite Elements (FE) whereas PLAXIS LE uses Limit Equilibrium (LE) analysis for slope stability. Both methods have their own advantages. The FE analysis methods have several advantages such as to model slopes with a degree of very high realism (complex geometry, sequences of loading, presence of material for reinforcement, action of water, laws for complex soil behavior) and to better visualize the deformations of soils in place.

FE analysis provides displacement pattern which may show potential and possibly complex failure mechanisms. The validity of the factor of safety obtained from LE analysis depends on locating the most critical potential slip surfaces. In complex conditions, it is often difficult to anticipate failure modes, particularly if reinforcement or structural members such as geotextiles, concrete retaining walls, or sheet piles are included. Once a potential failure mechanism is recognized, the factor of safety against a shear failure developing by that mode can be computed using conventional LE procedures.

An important input to the stability analysis for reinforced slopes (with geotextiles or soil nails and such) is the force in the reinforcement. The FE analysis can provide useful guidance for establishing the force that the reinforcement will experience under the complex loading sequences and groundwater conditions that can be established more accurately in FE analysis.  

To analyze slopes using the FE analysis, the strength reduction method is applied. This method is based on the reduction of the cohesion (c) and the tangent of the friction angle (tan φ) of the soil. The parameters are reduced in steps until the soil mass fails. The factor of safety of a slope in FE Shear strength reduction technique is similar to the one obtained in LE methods. One of the main advantages of strength reduction method is that the safety factor emerges naturally from the analysis without the user having to commit to any particular form of the mechanism a priori.

There is also plenty of literature of FE versus LE analysis which you can find related to your query. For example:

Farook, M. (2014). "Slope stability analysis- limit equilibrium or the finite element method?"
Hammah, G. (2015). "A comparison of Finite element slope stability analysis with conventional limit equilibrium investigation"

Hope this helps.

PLAXIS uses Finite Elements (FE) whereas PLAXIS LE uses Limit Equilibrium (LE) analysis for slope stability. Both methods have their own advantages. The FE analysis methods have several advantages such as to model slopes with a degree of very high realism (complex geometry, sequences of loading, presence of material for reinforcement, action of water, laws for complex soil behavior) and to better visualize the deformations of soils in place.

FE analysis provides displacement pattern which may show potential and possibly complex failure mechanisms. The validity of the factor of safety obtained from LE analysis depends on locating the most critical potential slip surfaces. In complex conditions, it is often difficult to anticipate failure modes, particularly if reinforcement or structural members such as geotextiles, concrete retaining walls, or sheet piles are included. Once a potential failure mechanism is recognized, the factor of safety against a shear failure developing by that mode can be computed using conventional LE procedures.

An important input to the stability analysis for reinforced slopes (with geotextiles or soil nails and such) is the force in the reinforcement. The FE analysis can provide useful guidance for establishing the force that the reinforcement will experience under the complex loading sequences and groundwater conditions that can be established more accurately in FE analysis.  

To analyze slopes using the FE analysis, the strength reduction method is applied. This method is based on the reduction of the cohesion (c) and the tangent of the friction angle (tan φ) of the soil. The parameters are reduced in steps until the soil mass fails. The factor of safety of a slope in FE Shear strength reduction technique is similar to the one obtained in LE methods. One of the main advantages of strength reduction method is that the safety factor emerges naturally from the analysis without the user having to commit to any particular form of the mechanism a priori.

There is also plenty of literature of FE versus LE analysis which you can find related to your query. For example:

Farook, M. (2014). "Slope stability analysis- limit equilibrium or the finite element method?"
Hammah, G. (2015). "A comparison of Finite element slope stability analysis with conventional limit equilibrium investigation"

Hope this helps.