In aldol reactions, it is apparently a general rule of thumb that (Z)-enolates give higher stereoselectivity (for the syn aldol product) than (E)-enolates do for the anti product. Quoting Carey & Sundberg, Advanced Organic Chemistry: Part B (5th ed.), p 69:

From these and many related examples the following generalizations can be made about kinetic stereoselection in aldol additions of lithium enolates.
(3) Z-Enolates are more stereoselective than E-enolates.

and the example given (taken from a Clayton Heathcock paper, J. Org. Chem. 1980, 45 (6), 1066–1081) is

Diastereoselectivity of aldol reaction

Evans also mentions this in his paper comparing the Cornforth vs polar Felkin–Anh models (Angew. Chem. Int. Ed. 2003, 42 (15), 1761–1765):

While Z and E lithium enolates are also formed with good configurational purity, the characteristic decline in syn/anti selectivity is observed, particularly with the E enolate.

Why is this the case? Or, why is $\Delta \Delta G^\ddagger$ for the (Z)-enolate larger (corresponding to greater selectivity) than for the (E)-enolate? I don't see anything that stands out when looking at the six-membered Zimmerman–Traxler transition states - within this model, the difference $\Delta \Delta G^\ddagger$ for both enolate geometries is simply the cost of putting a phenyl group axial instead of equatorial.

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    $\begingroup$ I can't get my thoughts into coherent sentences right now, but Heathcock (J. Org. Chem 1980, 45, 1066), and the nice (old) review In Comprehensive Organic Synthesis (Vol II, 190) give an okay explanation. tl;dr is that the aldol is kinetic but ΔG is so small that retro-aldol easily occurs to give a thermodynamic component. Keep in mind that Li-O bond long so TS energies essentially the same. Dave Evans' talks about this in his lectures and essentially says its a lie if the right enolate (boron not lithium) is chosen (its maybe more surprising that any Li aldol is selective). $\endgroup$ – NotEvans. Nov 7 '17 at 0:32
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    $\begingroup$ All the boron aldols I performed on ketones during my PhD thesis were fully stereoselective. I either only got a single product (I wanted anti) or I got two different anti-diastereomers stemming from the Felkin and the anti-Felkin attack. This supports what NotEvans has written about NotNotEvans. $\endgroup$ – Jan Nov 7 '17 at 9:51

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