From the Guidelines

Sitagliptin (100mg) and Glimepiride (2mg) have a similar effect on HbA1c reduction.

The evidence suggests that DPP-4 inhibitors, such as sitagliptin, and sulfonylureas, such as glimepiride, have a similar effect on HbA1c level when compared with each other 1.
A comparative effectiveness meta-analysis suggests that each new class of noninsulin agents added to initial therapy with metformin generally lowers A1C approximately 0.7–1.0% 1.
However, it is essential to consider the individual patient's characteristics, such as the presence of established ASCVD or indicators of high ASCVD risk, other comorbidities, and risk for specific adverse drug effects, as well as safety, tolerability, and cost, when choosing between sitagliptin and glimepiride 1.
The choice of medication added to metformin is based on the clinical characteristics of the patient and their preferences, and there is little evidence to support one combination over another 1.
Sitagliptin has been shown to be well tolerated and effective for glycaemic control, with a low risk of hypoglycaemia in patients with mild-to-moderate hyperglycaemia 1.
Glimepiride, an inexpensive and effective sulfonylurea, has been demonstrated to have a reassuring safety signal, with substantial benefits in terms of reductions in hypoglycemia 1.
In terms of HbA1c reduction, both sitagliptin and glimepiride can be effective, but the choice between them should be based on individual patient factors and preferences 1.

From the Research

The comparative effect of sitagliptin (100mg) and glimepiride (2mg) on HbA1c reduction has been studied in several clinical trials 2, 3, 4, 5, 6.
A study published in 2017 found that sitagliptin (100mg) achieved greater reductions in HbA1c compared to glimepiride (1-3mg) as an add-on therapy in patients with type 2 diabetes receiving insulin and metformin 3.
Another study published in 2019 found that sitagliptin (50mg) added to low-dose glimepiride (≤ 2 mg/day) resulted in a significant decrease in HbA1c level after 12 weeks, while glimepiride dose-up to 6 mg/day resulted in a significant decrease in HbA1c level after 24 weeks 2.
A study published in 2015 found that glimepiride and sitagliptin were equally effective in glycemic control, with the only difference being the frequency of hypoglycemic events, which was higher in the glimepiride group 4.
A study published in 2014 found that the difference in HbA1c reduction between sitagliptin dose-up and glimepiride dose-up was not significant, but glimepiride dose-up resulted in a significant decrease in HbA1c from baseline 5.
A study published in 2015 found that switching from glimepiride to sitagliptin resulted in a decrease in 2-hour postprandial plasma glucose levels and a significant reduction in hypoglycemic events 6.

Sitagliptin (100mg) may be more effective than glimepiride (2mg) in reducing HbA1c levels in patients with type 2 diabetes 3.
The combination of sitagliptin and low-dose glimepiride may be effective in reducing HbA1c levels and protecting pancreatic beta-cells 2.
Glimepiride and sitagliptin may be equally effective in glycemic control, but sitagliptin may have a lower risk of hypoglycemia 4.
The choice of sitagliptin or glimepiride may depend on individual patient characteristics, such as insulin resistance and baseline HbA1c level 6.