From the Guidelines

Lithium is more effective than Quetiapine for maintenance treatment in Bipolar Disorder (BD).

The evidence suggests that Lithium is a well-established treatment for BD, with a long history of use and a strong evidence base 1.
Quetiapine, on the other hand, is an atypical antipsychotic that has been shown to be effective in treating acute mania, but its efficacy for maintenance treatment is less well-established 1.
A double-blind, controlled trial found that Quetiapine plus Valproate worked better than Valproate alone for adolescent mania, but there is limited evidence comparing Quetiapine directly to Lithium for maintenance treatment 1.
The choice of medication should be based on evidence of efficacy, phase of illness, presence of confounding presentations, side effect spectrum and safety, patient's history of medication response, and patient and family preferences 1.
Lithium has been shown to be effective in preventing relapse in BD, with a study finding that withdrawal of maintenance Lithium therapy was associated with an increased risk of relapse, especially within the 6-month period following Lithium discontinuation 1.
In contrast, the evidence for Quetiapine is largely based on open-label trials and retrospective chart reviews, which have methodological limitations 1.
Therefore, based on the available evidence, Lithium appears to be a more effective option for maintenance treatment in BD, although Quetiapine may still be a useful adjunctive therapy in certain cases.

From the Research

The comparative efficacy of lithium (lithium carbonate) and quetiapine (quetiapine fumarate) for maintenance treatment in bipolar disorder (BD) has been investigated in several studies 2, 3, 4, 5, 6.
A review of evidence regarding quetiapine and lithium found that the two treatments have comparable efficacy in acute mania, and possibly greater efficacy for quetiapine compared with lithium in acute bipolar depression and in prevention of recurrent (particularly depressive) episodes 2.
The Bipolar CHOICE study, a randomized comparative-effectiveness study, found no overall significant differences in efficacy and safety/tolerability outcomes between lithium and quetiapine in patients with bipolar I or II disorder 3.

Quetiapine and lithium have overlapping but distinctive roles in different phases of bipolar disorder, with quetiapine possibly being more effective in acute bipolar depression and lithium being more effective in prevention of manic episodes 2.
A study found that continuation of quetiapine significantly increased time to recurrence of any mood, manic, or depressive event compared with switching to placebo, and switching to lithium was also more effective than placebo for the prevention of manic and depressive events 4.

Combination therapy including quetiapine and lithium was generally more effective than either agent alone in acute mania and bipolar maintenance, although adding lithium to quetiapine did not increase efficacy in acute bipolar depression 2.
The ARIQUELI study aims to evaluate the efficacy and safety of quetiapine monotherapy as maintenance treatment in bipolar I disorder compared with switching to placebo or lithium, and to investigate the potentiation of quetiapine with lithium or aripiprazole in nonresponders or partial responders 5.

A post hoc analysis of a double-blind study found that the effect of lithium in the maintenance treatment of bipolar disorder was associated with lithium plasma levels, with levels between 0.6 and 1.2 mEq/L being more effective than levels below 0.6 mEq/L or placebo 6.